STUDY GUIDE - EXAM 3 

What are the functions of mitotic cell division?

THE FUNCTIONS OF CELL DIVISION: 

What are the functions of mitotic cell division?

● The vast majority of cells in the adult body are not dividing

● To renew naturally used-up cells

● To replace cells after injury

● During Development

● During reproduction of single-celled organisms

What happens when cell division goes wrong?

● Developmental malformations

● Cancer = uncontrolled cell division

Two major problems to solve in cell division?

1. The actual feat of cell division

- Take one cell

- Make two out of it

- Both new cells need to have everything that the original cell had

What happens when cell division goes wrong?

2. Only getting cell division when and where it is beneficial to the organisms → regulation

CLICKER QUESTION:

If an adult human being loses an arm in an accident, it does not re-grow. Why not? - Developmental processes for growing limbs are not reactivated when a limb is lost later in life

What are the functions of mitotic cell division?

● To replace cells after injury

WHEN CELLS ARE NOT DIVIDING: INTERPHASE 

Cell Cycle​ = from the time a cell is formed (from the division of its parent cell), to the time it undergoes its own division into two cells

● Interphase 

Two major problems to solve in cell division?

If you want to learn more check out What are the three or four “powers” commonly associated with esp?

○ G1 phase = Cell is growing

○ S Phase = Synthesis phase

■ Prep to make two daughter

cells

■ All of the DNA in the cell is

being duplicated

○ G2 Phase = Growth

■ Prepared to enter into mitotic cycle

■ More growth

● Enter Mitotic Phase 

○ Cytokinesis

○ Mitosis

○ Divide

** Some cells (neurons) will ever go through this whole cycle

- Cells can go from G1 to G0

CLICKER QUESTIONS:

Which of the following statements about INTERPHASE is FALSE?

- Very few of our cells are in interphase at any one time

Who has the highest percentage of cells in S phase?

- A fetus

- The highest percentage is in a fetus because most of the cells are undergoing cell division for growth, therefore lots of them in S phase

THE MITOTIC PHASE:

Mitosis + Cytokinesis

Mitosis: Don't forget about the age old question of What is the doppler effect?

- Prophase

- Prometaphase

- Metaphase

- Anaphase

- Telophase

Mitosis:

● When the cell divides

● Has two parts:

○ Mitosis= division of nucleus and nuclear material

○ Cytokinesis = division of rest of cell

- Duplication occurs in S phase

- Split up in mitosis

- Sister chromatids = two copies of one chromosomes

CLICKER QUESTION:

Suppose that at the end of G2 phase, a skin cell (in some species a mammal) has 16 chromatids total. How many chromosomes will each cell produces from this cell have after mitosis and cytokinesis are complete?

- 8

- 16 chromatids and originally had 8 single chromosome

- It will separate to 8 in each cell

- With mitosis were making identical pairs Don't forget about the age old question of What is symposium?

Suppose that at the beginning of S phase, a skin cell has 8 chromosomes total. How many chromosomes will each cell produced from this cell have after mitosis and cytokinesis are complete?

- 8

- Before S phase means the DNA has not duplicated We also discuss several other topics like Why would cerebral asymmetry evolve?

- 16 Chromatis once it duplicated

- Then those divide and you get 8

MITOSIS PHASES: (video to explain on Mastering Bio > study area > ch 9 > Bioflix: Mitosis)

Prophase:

● Development of early mitotic spindle

● Very first stage of mitosis

Prometaphase:

● Nuclear membrane breaks down

Metaphase:

● Chromosomes line up in the middle of the cell

Anaphase:

● Sister chromatids are separated, pulled by spindle

Telophase:

● Nuclear membrane reforms

● Cytokinesis can begin at same time

Put the following processes in the right order from start to finish from interphase through mitosis: 1. Sister chromatids separate

2. DNA is replicated

3. Chromosomes condense

4. Chromosomes line up in the middle of the cell

5. Nuclear envelope dissolves

D) 2, 3, 5, 4, 1

Suppose a scientist “tricked” a cell to ski straight from G1 phase (to interphase) to the Mitotic phase. Which of the following processes would be IMPOSSIBLE for the cell during mitosis? E) Separation of sister chromatids

OUTLINE:

A. Cell Cycle and Mitosis

B. Regulation of the Cell Cycle If you want to learn more check out What are the efficiency effects of monopoly?

a. Checkpoints

b. The cell cycle is regulated by proteins in the cytoplasm

c. Cancer result from a loss of cycle control

1) The Concept of Checkpoints 

● Mitotic checkpoints: move on to the next phase only if conditions are right ● Prevents sick or precancerous cells from dividing and maintains proper cell number ● Checkpoint sensing mechanisms monitor:

○ Progression of the current phase

○ DNA integrity

○ Cell density

○ Cell anchorage

2) Regulation of cell cycle If you want to learn more check out How to prepare for your interview ?

● Concentrations of proteins in the cytoplasm

regulate the cycle

○ Particularly with the G2 checkpoint

● Cyclins are ONE EXAMPLE → there are two

checkpoints and other molecules at work

Cyclin A and checkpoints:

If anything is out of whack, the cell cycle is stopped by:

- Increased rate of cyclin degradation

- Inhibiting interaction of cyclin and CDK

- Both of these processes act to prevent the cell from crossing the checkpoint between G2 and the Mitotic Phase

Which of the following could be ways that keep the cell from passing the G2 checkpoint? A. Degrading (breaking down) cyclin proteins during G2

B. Producing proteins that inhibit the formation of “mitosis promoting factor” C. A and B

Cancer 

● Cancer results in a loss of control over the cell cycle

● Chemotherapy halts the cell cycle

○ Cancer cells do not have density dependant inhibition

How cancer arises - carcinogens

● Multi hit or multi-step hypothesis: mutations in several cell cycle regulators ○ Mutations in genes which, when functional, maintain the checkpoints ● Thus exposure to anything “mutagenic” (= mutation causing) increases cancer risk but does not always “give you cancer”

○ UV radiation

○ Cigarette smoke

○ Radioactivity

Chemotherapy drug stops the cell cycle. Why are stomach upset, hair loss, and immune system depression all common side effects of chemotherapy?

B) Compared to other tissues, cells in these tissues are marked by higher rates of cell division

Chemotherapy stops the cell cycle → side effects (after all, cancer cells are not the ONLY dividing ells in a person's body)

● The most affected tissues have the most cell division

○ Hair, nails

○ Skin

○ Bone Marrow

○ Gut

Chemotherapeutic drugs​ work by stopping cells from diving. Many were first isolated from plant.

● Many chemotherapeutic drugs were first isolated from plants

● Vinca Alkaloids from the Madagascar periwinkle bind to spindle building blocks preventing their assembly

Which process in the cell cycle is most directly blocked by Vinca Alkaloids? C) attachment of spindle of chromosomes

● Podophyllotoxin from American and Himalayan mayapples prevents DNA replication

In which part of the cell cycle would a cell be stalled by podophyllotoxin

C) S Phase

Meiosis and Sexual Life Cycles

OUTLINE:

1. Heredity

2. Sexual Reproduction

3. A detailed look at meiosis

4. Where does variation come from?

Biological Heredity 

Biologically speaking, what part(s) of a parent is.are physically duplicated and passed on to offspring?

A) Genes

Heredity = ​passing on copies of genetic materials

This is absolutely essential for:

- Reproduction

- Evolution

- The persistence of populations and traits that enable their members to survive

Two Basic Ways to transmit genetic material from parent to offspring: 1. Asexual reproduction:

- One parent clones itself to make offspring

2. Sexual reproduction:

- Two individuals contribute genetic material to offspring

Sexual Reproduction 

- Pairs

- Members of a pair are called “homologous chromosomes

Homologous Chromosomes

● What do chromosomes contain?

○ Long DNA molecule + accessory proteins

○ The long DNA molecule contains “instructions”

○ Instructions come in discrete units that we call genes (each chromosome has MANY genes)

○ Each gene is located at a specific location on a specific chromosome

Homologous Chromosomes carry the same genes 

● Cells that contain 2 Sets of homologous chromosomes are called diploid ● Cells that have only one of each kind of chromosome are haploid 

Notation:

- Chromosomes number in diploid state = “2n”

- Chromosomes number in haploid state = “n”

What is ‘n’? 

● n is a umber

● n is specific to a species

Definition: n = the number of chromosomes a haploid cell contains

- Thus 2n is the number of chromosomes a diploid cell contains

- ( ‘2n’ means ‘2 x n’)

EX:

- Dog diploid cells have 78 chromosomes

- So for dogs 2n = 78, thus n = 39

- Dog gametes (dog sperm and dog eggs; haploid) thus have 39 chromosomes each

- Cat diploid cells have 38 chromosomes

- 2n = 38, n = 19

- Thus cats have 19 chromosomes each

Most of the cells in your body are diploid. How did they become this way? A. Each of us is a product of sexual reproduction, having one set of genes from our mother and one set of genes from our father

Below is a picture of a child's skin cell, showing 6 of the cells chromosomes, all of which have been replicated. 3 chromosomes were inherited from the child's mom, and 3 from the child's dad. Which of the following best describes a pair of homologous chromosomes?

C) Two chromosomes, one inherited from each parent, that have the same genes as each other

What makes two specific chromosomes homologous?

Fertilization and Meiosis alternate 

Life Cycle - ​from conception of an organism to the time is reproduces

Meiosis - ​the division of a diploid cell leading to the eventual production of haploid cells ** Alternation of haploid/diploid

Which of the following is TRUE about Meiosis and sexual life cycles?

b) meiosis halves the number of chromosomes; fertilization restores the diploid number

Outline: 

1. Inheritance

2. Sexual Reproduction

3. A detailed look at meiosis

4. Where does variation come from?

Meiosis: 

A cell at the very beginning of meiosis has a total of 4 chromatids. What is “n” for the cell?

A) 1

At the very beginning of meiosis, a cell in your ovary/testis has 46

chromosomes. At the end of meiosis I (but before meiosis II), this cell will have given rise to two cells that are each ________ and each have ________ chromosomes and ________ chromatids.

D.) Haploid, 23, 46

MEIOSIS​:

Early in Meiosis 1:

● Homologous chromosomes loosely pair up, aligned gene by gene ● In crossing over, non-sister chromatids exchange homologous DNA segments Middle of Meiosis 1:

● Pairs of homologous chromosomes line up in the middle of the cell, with one chromosome facing each pole

● Spindle attaches to chromosomes

What is MAJORLY different here from what we

saw in mitosis??

What is MAJORLY different here from what we

saw in mitosis?

With first cell division complete, Meiosis 1 ends

● Meiosis 2 is very similar to mitosis

● At the end = 4 daughter cells, all

haploid, all genetically different

Mitosis vs Meiosis:

Mitosis (diploid and haploid) 

DNA replication

● Occurs during interphase before mitosis begins

Number of divisions

● One, including prophase prometaphase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes

● Does not occur

Number of daughter cells and genetic composition

● Two, each genetically identical to the parent cell, with the same number of chromosomes Role in the animal or plant body

● Enables multicellular animal or plant (gametophyte or sporophyte) to arise from a single cell

● Produces cells for growth, repair, and in some species, asexual reproduction ● Produces gametes in the gametophyte plant

Meiosis (diploid only) 

DNA replication

● Occurs during interphase before meiosis 1 begins

Number of divisions

● Two, including prophase prometaphase, metaphase, anaphase, and telophase Synapsis of homologous chromosomes

● Occurs during prophase 1 along with crossing over between nonsister chromatids; resulting chiasmata hold pairs together due to sister chromatid cohesion

Number of daughter cells and genetic composition

● Four, each haploid (n); genetically different from the parent cell and from each other Role in the animal or plant body

● Produces gametes (in animals) or spores (in the sophyte plant)

● Reduces number of chromosome sets by half and introduces genetic variability among the gametes or spores

WHERE DOES VARIATION COME FROM? 

Sources of genetic variation 

● Mutations (changes in an organisms DNA) are the original source of all genetic variation ● Mutations create different versions of genes called alleles

Homologous Chromosomes:

- Same gene, different ALLELES

● The behavior of chromosomes during meiosis and fertilization reshuffles alleles and chromosomes every generation

● The behavior of chromosomes during meiosis and fertilization reshuffles alleles and chromosomes every generation

● Four mechanisms contribute to genetic variation in the offspring of sexual reproduction: 1. Mutations ​= change in DNA sequence

2. Independent assortment of chromosomes

3. Crossing over

4. Random fertilization

Independent assortment: 

● Homologous pairs of chromosomes orient randomly during Meiosis 1 ○ Maternal and paternal homologs assort into daughter cells independently of the other pairs

- 4 possible assortments of

chromosomes in the gametes

Meiosis:

Suppose you were building a picture or a physical model of a cell in order to demonstrate the principle of independent assortment. What is the SMALLEST number of chromosomes you could put inside your model cell in order to do this? Why?

“2nd Rule”: the number of possible chromosome sorting combinations = 2n (remember in = the haploid number)

- For humans (n=23), there are = 8,388,608 possible combinations of chromosomes!

If an organism has a diploid number of 6 chromosomes, how many different combinations of maternal and paternal chromosomes are possible in its gametes?

B) 8

Crossing Over (early in Meiosis 1) 

● Homologous chromosomes pair up gene

by gene and exchange homologous

segments

● This combines alleles that originated from

two parents into a single chromosome

First Cell Division = MEIOSIS 1

Second Cell Division = MEIOSIS 2

Random Fertilization 

Female: 8.4 million possible gametes

Male: 8.4 million gametes possible

= Possibility of 70 trillion offspring!!

Which of the following can NOT contribute to genetic variation in offspring arising from sexual reproduction?

e) mutations in DNA caused by UV rays hitting an a parent’s dividing skin cells

During which of the following does the separation of sister chromatids occur? e) Mitosis and Meiosis II

During which of the following does the separation of homologous chromosomes occur? b) Meiosis I only

OUTLINE​:

1. The work of gregor Mendel

a) The scientific method

b) Mendel's explanatory framework

2. Probability and genetic outcomes

3. Ah, if only it were so simple: complication on genes and traits

Blending Inheritance: 

Blending Inheritance​ = Traits of offspring will be an average of the traits of the parents.. Problem: Loss of variation → predicts a population uniform in its traits

Scientific Method: 

In science, what is the difference between an

“observation” and a “prediction”?

C) Predictions are what you expect is a

given hypothesis is correct; observations are what

you actually find or measure

In science, what is required to test a hypothesis?

D) gathering data and checking whether the

data are consistent with or contradict

predictions of the hypothesis

In science, what does it mean when observations contradict the predictions of a hypothesis? B) we should consider alternative hypotheses (if we weren't already)

MENDEL: 

Good experimental design:

1. Characters with discrete variants (ex - purple vs white flowers; “either-or” 2. Started with true breeding varieties

“True Breeding”

- Offspring produced from two “true-breeding” flowers of the same color would always have the same color flower as their two parents

A purple flower is crossed with a white flower.

What does the hypothesis of blending inheritance

predict that their offspring should look like?

C) Purplish-white

When purple

flowers are

crossed with each other, as in the F1 generation, what

does the hypothesis of blending inheritance predict

their offspring (the “F2 generation”) should look like?

A) All Purple

MENDEL’S EXPLANATORY FRAMEWORK 

“Explanatory Framework​” - multiple internally consistent hypotheses that together explain general phenomena of interest. In science this is also called a theory​.

Mendel’s Theory: 4 related hypotheses

- These can be related to what we know about genes and chromosomes! 1. Alternative versions of heritable “particles” (ex - different alleles of the same gene) account for variations in inherited characters

- Opposite of blending inheritance = particulate inheritance

2. For each character an organism inherits two alleles, one from each parent 3. If the two alleles at a locus differ, then one (the dominant allele) determines the organism's appearance, and the other (the recessive allele) has no noticeable effect on appearance

4. “Law of segregation”​ = the two alleles an individual possesses for a heritable character separate (segregate) during gamete formation and end up in different gametes

Which of Mendel’s four hypotheses can, on its own, directly explain why there are NO white flowers in the F1 generation and why the purple F1’s look just as purple as the purple plants in the parental (“P”) generation?

C) If the two alleles at a locus differ, then one (the dominant allele) determines the organism's appearance, and the other (the recessive allele) has no noticeable effect on appearance

Here are Mendel’s four hypotheses.

1. Alternative versions of heritable “factors” (i.e., alleles) account for variations in inherited characters

2. An organism inherits two alleles, one from each parent

3. If the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance.

4. The “law of segregation”

Though Mendel didn’t know about “ploidy” per se, two of these hypotheses together constitute the idea of alternations between haploid and diploid in sexual life cycles. Specifically, #______ is similar to saying meiosis makes haploid gametes, and #_______ is similar to saying fertilization makes a diploid zygote.

C) 4, 2

Mendel did not know about genes or chromosomes per se. However, in modern terms, Mendel’s “law of segregation” can be phrased as stating that the TWO alleles an individual possesses for a given gene SEPARATE during gamete formation.

A) Diploid

OUTLINE: 

1. The work of Gregor Mendel

2. Probability and Genetic outcomes

3. Complications on genes and traits

Mendel's Theory: 4 Related Hypotheses 

1. Alternative versions of heritable “factors” (i.e alleles) account for variations in inherited characters

2. For each character an organism inherits two alleles, one from each parent 3. If the two alleles at a locus differ, then one (the dominant allele) determines the organism's appearance on the other (the recessive allele) has no noticeable effect 4. Law of segregation

Mendel did not know about genes or chromosomes perse. However, in modern terms, Mendel's “law of segregation” can be phrased as stating that the TWO alleles an individual possesses for a given gene Separate during gamete formation.

What happens to cells at the chromosomal level during segregation?

B) Haploid Cells are arising from diploid ones

Mendel did not know about genes or chromosomes perse. However, in modern terms, Mendel's “law of segregation” can be phrased as stating that the TWO alleles an individual possesses for a given gene Separate during gamete formation.

Mendel's law of segregation finds its mechanistic basis most clearly in the behavior of chromosomes during?

C) Meiosis I

Probability and Genetic Outcomes: 

- Mendel’s “Law” of segregation is used to construct a “Punnett square”​ - This simple square tells you the expected frequencies of genotypes and phenotypes from a particular cross

2 alleles x 2 alleles = 4 outcomes, 1:1:1:1

- only 3 distinct genetic types, or genotypes 1:2:1

- only 2 distinct traits, or phenotypes, 3:1

Practice making and interpreting a Punnet square. Suppose there are two alleles of a gene, called A and a, and A is dominant to a. In a cross, Aa x Aa, among the offspring there are _____ possible genotypes and _____ possible phenotypes.

B) 3, 2

In the previous question, the expected ratio of the different possible phenotypes in the offspring of that cross is

C) 3:1

Genetic Detectives 

● How can we figure out the Genotype of an unknown purple flower?

● Which of these crosses would you do to figure out the genotype of a mystery purple flower?

= C) pp

Note​:

- With two alleles, P and p, there are three possible genotypes: PP, Pp, pp - PP and pp are homozygous genotypes; individuals with these genotypes are called homozygotes

- Pp is the heterozygous genotype = genotype carried by heterozygotes

What about multiple characters? Are they inherited together or separately? ● For the purpose of ex, consider the following two characters:

1. Seed Color:

- Possible phenotypes = Yellow OR Green

- Yellow is dominant to green

2. Seed Shape:

- Possible phenotypes = Round OR Wrinkled

- Round is dominant to wrinkled

Possible alleles of a “second gene”

Possible alleles of a “seed shape gene”

What about multiple characters? Are they inherited together or separately? P generation of experiment:

- True breeding yellow round crossed with true breeding green wrinkled

What are the genotypes of (1) yellow round and (2) green wrinkled in the P generation? (use information/letters from previous slides)

C) Yellow round = YYRR Green Wrinkled = yyrr

Why does each parent in the P generation make only one kind

of gamete?

C) because each parent is homozygous for both

genes

Suppose that two F1 individuals are crossed. Consider two mutually exclusive hypotheses about inheritance: 

1. Strict dependent assortment

- Inherited allele combinations are always preserved in the gametes an individual produces

2. Independent assortment

- All possible combinations of inherited alleles of different genes are equally likely in an individual's gametes

Under the hypothesis of strict dependent assortment, which of the following types of gametes would an F1 individual produce?

YR Yr yR yr

= YR and yr

Mendel’s “law” of independent assortment = alleles for each character segregate independently during gamete formation

Given what YOU know about the relationship between genes and chromosomes (which Mendel did NOT), this “law” would likely be violated for two genes that:

D) are on the same chromosome

Complications on Genes and Traits: 

● One gene → One character

● One allele → One phenotype

● Two alleles of each gene, one completely dominant, the other recessive

Patterns of inheritance different from those discussed so far can be caused in many ways. Just to name a few: 

a) Lack of complete dominance by one allele

b) A gene has more than two alleles

c) A gene produces multiple phenotypes

d) Multiple genes affect a single phenotype

e) Environmental circumstances affect the phenotype

More about dominance: 

- A dominant allele does not subdue a recessive allele; alleles do not interact

Misconceptions about dominance: 

- Dominant alleles are NOT necessarily the most common alleles

- Dominant alleles are not necessarily the “best”

Synthesis: Genes on Chromosomes 

● Mitosis​ - Replication and division of chromosomes to multiply cells ● Meiosis​ - Replication and division of chromosomes to make gametes ● Mendel​ - The particulate theory of inheritance

OUTLINE​ - Genes on Chromosomes

1. Mendelian inheritance has it's basis in the behavior of chromosomes a. Hopefully a review

2. Genes on chromosomes: Sex chromosomes

3. Mapping Genes’ locations on chromosomes

Mendel's model and chromosome behavior

Genes on chromosomes: Sex chromosomes 

● Humans and many other species have chromosomal sex determination ● In the human system, females usually have two “X” chromosomes, males usually have one “X” and one “Y”

● Patterns of inheritance in mammals (and other XY systems)

From Female Parent

From Male Parent

Allele on X Chromosome (“X-linked”)

Passed on to other sons or daughters with probability ½

Passes on ONLY to

daughters with probability 1

Allele on Y Chromosome (“Y-linked”)

Typically not possessed by females

Passed on ONLY ro sons ith probability 1

Disorders caused by recessive, sex-linked alleles should be most commonly expressed in: B) female birds (ZW) and male humans (XY)

Thomas Hunt Morgan 

● Embryologist

● Study Organisms: the fruit fly

The Fruit Fly:

● They breed at a high rate

● A generation can be bred every two weeks

● They have only four pairs of chromosomes

● Males are XY and females are XX

● By studying them carefully, rare “mutant” phenotypes could be identified that were different from the normal “wild type”

One of Morgan's experiments:

● Character = eye color

● Phenotypes = red or white

● Parental generation = true breeding lines

● White allele is recessive

The best explanation for the pattern of inheritance seen in the F2 generation is:

B) The eye color gene is sex-linked, on the X chromosome

Given the conclusion from Morgan's work: If morgans

parental generation had been white-eyed Females and

red-eyed Males, the ____ of the males and _____ of the

females in the F1 generation would have have white eyes

D) all; none

Genes on Chromosomes: Gene linkage and mapping

EX:

- Morgan crossed field to study the characters of body color and wing size - Genes for both located on autosomes

What are the parental and nonparental combinations

of alleles and junior can produce in his gametes?

D) parental = AB, ab; non-parental = Ab, aB

Terminology:

- Combinations of alleles that are non-parental are called “recombinant” - Dihybrid​ = a hybrid that is heterozygous for alleles of two different genes

Why would some genes be inherited neither completely together nor completely independently? ● Gene Linkage

○ Each chromosome has hundreds or thousands of genes

○ Genes located on the same chromosome that tend to be inherited together are called linked genes

Recombination of Linked Genes 

● Morgan discovered that:

○ Body color and wing phenotype were often inherited together: offspring usually had the same combinations of phenotypes as one of the parents

■ Genes on same chromosome

○ BUT, linkage was incomplete, as evident from recombinant phenotypes (combos different from parental types)

● Some process must sometimes break the physical connection between genes on the same chromosome

Information on recombination frequencies can be used to create a “genetic map” - How?

- Assume that genes that are farther apart will show higher recombination frequencies

Gene Mapping: some useful terms 

● Linkage map: genetic map of a chromosome based on recombination frequencies ● Notion of distance: amp units; one map unit= 1 % of recombination frequency ● Map units indicate relative distance and order, not precise locations of genes

You conclude that:

In experiments with two fruit fly characters like those we just learned about, suppose that the ratio of the two parental types and the two recombinant types was 1:1:1:1. What would the recombination frequency be?

In experiments with two fruit fly characters like those we just learned about, suppose we observe a recombination frequency of 50%. What could explain this?

● the two genes for the two characters are far apart on the same chromosome, and thus crossing over between them happens very frequently

● the two genes for the two characters are located on different, nonhomologous chromosomes

● the two genes for the two characters are located close together on the same chromosome

● A and B are both good answers

● A ,B, and C are all good answers

Cautionary notes on linkage maps: 

● Genes that are far apart on the same chromosome can have a recombination frequency near 50%

● They behave as it they are unlinked

Considering two characters (ex - body color and wing type)

the maximum value we should ever observe for

recombination frequency is:

● sloppy experimentation

● the occurrence of two crossovers between vg and b reduces the observed frequency of recombination for those two genes

● the occurrence of two crossovers between cn and each of the other two genes increases the observed frequency of recombination for those pairs of genes

● meiosis shuffles the relative locations of genes in everygeneration

Why does independent assortment strictly apply ONLY to genes on NONhomologous chromosomes? ... or in other words ...

Independent assortment does NOT (always) apply to genes on the same chromosome. Why not?

● Allele combinations on a chromosome can never be broken up

● A combination of alleles on one chromosome is less likely to be broken up than a combination of alleles from multiple chromosomes

● Non-homologous chromosomes pair up during meiosis

● Meiosis makes it impossible to create non-parental combinations of alleles in gametes.

Gene Mapping: some useful terms 

● Linkage map​ = genetic map of a chromosome based on recombination frequencies ● Notion of distance: map units; one map unit = 1% recombination frequency ● Map units indicate relative distance and order, not precise locations of genes

Continuing with genetics … 

● Genes and Inheritance

● The molecules that contain our genes

● How info from these molecules comes to life

DNA INTRO OUTLINE: 

1. DNA = the molecule of heredity

2. The organization of DNA in chromosomes

Why is DNA so important?? 

● Quite simply, it is the information in your DNA that directs the program that takes materials (sugars, fats, amino acids, etc.) and turns them into YOU

● DNA is also how we pass this program to our descendants

● How does this all work??

○ Up until the early 1950s:

■ Known:

■ Much about cell chem (cells contain DNA and proteins)

■ The work of Mendel, Morgan and many others → any findings suggesting chromosomes/DNA contain hereditary info

■ Unknown/unproven:

■ Is hereditary info stored in proteins or DNA or RNA or ?

■ What is the structure of DNA

● We know that:

○ DNA is a double stranded molecule

○ The two strands are complementary, and together wind around in the form of a double helix

● DNA​ is a deoxyribonucleic acid

● A nucleic acid is a polymer made of nucleotide monomers

● A nucleotide​ consists of:

○ A nitrogenous base

○ A sugar

○ A phosphate group

● The sugar and phosphate group are the SAME for all nucleotides

● The base is what varies

● The sequence of nucleotides with different bases forms the code containing all hereditary info

● There are only 4 nitrogenous bases → four possible nucleotides

● Different sequences of the four possible nucleotides make up the different genetic code of every living organism

1950: Erwin Chargaff 

● Looking at chemical composition of DNA in any particular species:

○ % adenine (A) = % thymine (T)

○ % guanine (G) = % cytosine ©

○ These equalities are known “Chargaff’s Rule”

○ EX: Human DNA

■ 30% of nucleotides are A, and 30% are T and likewise

■ 20% G and 20% C

○ Many clues about chemical composition → but what was the structure?

Suppose that 22% of an organism’s genome is the nucleotide guanine (G). What else do you know about the organism?

D. It must have 22% C, 28% A, and 28% T

Building evidence about the structure of DNA…

- If an unpublished report, Franklin postulated that

1. DNA had a sugar-phosphate backbone

2. Around the outside of a double stranded molecule

3. With nitrogenous bases arranged in the middle

● The strands of DNA run in an antiparallel manner

○ The phosphate group in a single nucleotide is attached to the 5’ carbon atom of the sugar

● Adjacent nucleotides are linked phosphate to 3’ carbon atom → this forms the “backbone”

● Antiparallel arrangement allows proper base pairing

● The phosphate group in a single nucleotide is attached to the 5’ carbon atom of the sugar

● Antiparallel strands form a double stranded molecule

- Because each nucleotide has a top and a

bottom, each single strand of DNA has a 5’

and a 3’ end

Which of the following correctly represents base pairing in a DNA molecule?

Why was elucidating the structure of DNA such a major breakthrough? 1. Suggested how different organisms could have different genes: they have different nucleotide makeups…

2. Suggested a mechanism for replication --? Each strand can serve as a template for a new strand containing exactly the same info!

a. Duplication of info is essential for heredity!

● The copying of DNA is remarkable in its speed and accuracy:

○ In a human cell:

■ 6,000,000,000* base pairs replicated in a few hours

■ Error rate = 1 / 10,000,000000 nucleotides

What’s in all that DNA? 

● “Only” about 20,000 - 30,000 genes

● Only 1.5% of your DNA codes for proteins/enzymes!?

● The rest is regulatory sequences and lots of non-coding or “junk” regions

The organization of DNA in chromosome 

Suppose a cell has 6 chromosomes. At the beginning of G1 phase, _____ DNA molecules are present in this cell. At the beginning of G2 phase, _______ DNA molecules are present in this cell. (NOTE: a DNA molecule, even though it has two strands, is considered to be ONE molecule.)

During Replication of DNA 

(happens during S phase of Interphase)

Suppose a cell in some species of animal has 20 chromosomes. If this cell is in G2 phase, it must have _____ chromatids and ______ double-stranded DNA molecules.

Watson and Crick

“semi-conservative” model

- Basic principle: base

pairing with a template

strand

In DNA replication, what happens to the original, “parent” DNA molecule (parent molecule = the double-stranded DNA molecule present before replication starts)?

a) its two strands stay together, and the copy of the parent molecule consists of two newly synthesized daughter strands

b) the original molecule serves as a template which is eventually destroyed once two new daughter molecules are synthesized

c) the two strands in the original molecule separate, with each serving as a template for a newly synthesized strand

d) none of the above

e) all of the above

When a new strand of DNA is being synthesized, what determines its sequence? a) the sequence of the template strand

b) the rules of complementary base pairing

c) hydrogen bonding

d) all of the above

Based upon the semiconservative model, which is true about the DNA molecules contained by sister chromatids present in G2 phase?

a) Only one sister chromatid contains a newly synthesized DNA molecule; the other sister chromatid has the original DNA molecule

b) Each sister chromatid has a double-stranded DNA molecule

c) Each sister chromatid has a strand of DNA from the original DNA molecule paired with a newly synthesized strand

d) a and b

e) b and c

DNA replication in Eukaryotes

DNA replication is not a trivial undertaking

Even before actual replication begins:

1. Open the replication bubble → separates the two strands

2. Unwind the helix!

- Specialized enzymes heep the process running smoothly

DNA Polymerases:

1. Match the right monomers with the template

2. Catalyze formation of new sugar-phosphate

bonds

3. Add 50 (eukaryotes) to 500 (bacteria) bases

per second

Directionality:

1. DNA polymerase “reads” the parental

template from 3’ to 5’

2. DNA polymerase synthesizes the new strad by building onto the 3’ end of the primer or newly made strand → it builds the new strand 5’ to 3’

● The lagging strand is synthesized as a series of fragments

○ Subsequently joined together by DNA ligase

Based upon the semiconservative model, which is true about

the DNA molecules contained by sister chromatids present in

G2 phase?

E) B and C

B) each sister chromatid has a double-stranded

DNA molecule

C) each sister chromatid has a strand of DNA from

the original DNA molecule paired with a newly synthesized

strand

In DNA replication, the difference between the “leading” strand and the “lagging” strand is that C) synthesis of the lagging strand begins after synthesis of the leading strand is complete

Consequences of Linear Chromosomes and the requirements of DNA polymerase - What happens at the ends of chromosomes?

- Today's featured “organism”: your

telomeres

What happens at the ends of 

chromosomes? 

- Gradual shortening / erosion of DNA in

most of our cells as divisions go on throughout our lifetimes

- Fear not! DNA molecules have

telomeres at the ends

Does this shortening represent a faulty operation of our biology?

- Not necessarily: It may be a mechanism that can limit divisions of cancer cells!