GENE Exam 2 Study Guide:
Chromosomes and Cellular Reproduction:
• Eukaryotic Features:
o Eukaryotic cell nuclei have a nuclear envelope that surrounds the nucleus.
o Their DNA associates with histones, a protein, in order to tightly pack and form a chromosome.
▪ The DNA and histone proteins together are called chromatin. ▪ Histone protein aid in accessibility of DNA to enzymes and
proteins when going through replication or transcription.
▪ They also help the DNA to fit inside the nucleus.
• Cell reproduction in eukaryotes:
o The genetic information needs to be copied, be separated, and divide.
o Each species has a certain number of chromosomes per cell. ▪ Humans have 46.
o A pair of the same chromosomes is a homologous pair.
o Diploid cells have two sets of chromosomes, haploid have one. ▪ Reproductive cells are haploid.
o Chromosome features:
▪ A centromere is the constricted region of the chromosome. • It is the attachment point for spindle microtubules – the
things that move the chromosomes during cell division.
• It is formed by a kinetochore.
▪ Telomeres are the DNA sequences at the tips of chromosomes. • They protect and stabilize the ends.
We also discuss several other topics like What does neolithic mean in art?
▪ Origins of replication are where DNA synthesis starts.
o When a cell gets ready for division, chromosomes will replicate. ▪ These replications are held together at the centromere and are called sister chromatids.
▪ Each sister chromatid is a single molecule of DNA.
o The cell cycle:
▪ its regulated at checkpoints that will either allow or stop cell progression.
▪ There are two phases of the cell cycle:
• Interphase – where the cell grows, develops, and functions.
• M (mitotic) phase – where active cell division takes place.
o This includes mitosis – nuclear division and
cytokinesis – cytoplasmic division.
▪ Interphase is divided into three stages:
• G1, S, and G2.
• G1: We also discuss several other topics like Why are social determinants of health important?
o The cell grows.
o There’s a checkpoint at the end of this stage.
o If the cell doesn’t pass this phase it will enter a nondividing stage = G0. The cell will reenter the If you want to learn more check out What are the limits of individual rights and liberties?
cycle when ready.
o Stands for DNA synthesis where the chromosome is duplicated.
o After this phase, each chromosome will be two sister chromatids.
o The cell grows some more.
o There is a checkpoint at the end of this phase.
o It will only pass the checkpoint if the DNA is
replicated and undamaged. We also discuss several other topics like What is the study of how people make choices under scarcity and the
results of these choices for society?
▪ The M phase:
• This is where the sister chromatids will separate, and the cell will divide.
• There are 6 phases:
▪ Chromosomes condense.
▪ Mitotic spindle and microtubules form.
They grow out of centrosomes that migrate
to the opposite sides of the cell.
▪ Nuclear envelope disintegrates.
▪ Spindle microtubules connected to
▪ Chromosomes align on the metaphase
▪ Sister chromatids separate to become
▪ They migrate to separate poles.
▪ Nuclear envelope reforms
▪ Condensed chromosomes relax.
o Cytokinesis: If you want to learn more check out Why do we study human anatomy and physiology?
▪ Cytoplasm divides
▪ The cell cycle takes a single cell and makes two cells that have identical genetic information. One cell with a number of chromosomes to two identical cells with the same number of chromosomes.
▪ Sexual reproduction
▪ Increases genetic variation
▪ Differences and similarities from mitosis:
• Mitosis is a singular division and meiosis is two division. • Mitosis results in the same number of chromosomes per cell as the original, but meiosis results in half the
number of the original. If you want to learn more check out What are the four basic objectives of tort law?
• Mitosis results in genetically identical cells, but meiosis does not.
• Meiosis includes G1, S, and G2 as well, but has meiosis I and II.
o Meiosis I reduces the chromosome number in half
and meiosis II keeps the number the same. 2n -> n
• These are the same as mitosis, but meiosis I starts with homologous chromosomes that will line up along the
plate (each pair lines up with each other along the plate) in metaphase and the pairs are separated from each
other in anaphase. This results in half the number of the
original chromosomes as meiosis I. Meiosis II proceeds
similar to mitosis.
▪ Genetic variation in meiosis:
• Crossing over – exchange of genetic info between non
sister chromatids (chromatids from different
o This is the basis for recombination.
o Only happens in meiosis I.
• Random separation of chromosomes in anaphase I.
▪ Pictorial representation of a family history and outlines the inheritance of one or more characteristics.
▪ Autosomal recessive traits:
o Appear with equal frequency in both sex
o Only appear when a person is homozygous recessive, meaning that you can be a carrier.
o If the trait is rare then the parents of affected offspring are heterozygous, and people outside of the affected family are assumed to be homozygous dominant.
o Skips generations.
▪ Autosomal dominant traits:
o Equal frequency between sexes.
o Do not skip generations, if affected they will have an affected parent. ▪ This will not hold true in cases of new mutations of incomplete penetrance though.
▪ X-linked recessive traits:
o Appear more in males than females.
o Affected males will usually have a carrier mother.
o If a woman is a carrier, half her sons will most likely be affected and half of her daughters will be unaffected.
o They cannot be passed father to son (because the son will be receiving the Y chromosome from the dad and not an X).
o If a woman is affected, she will be homozygous for the trait. ▪ X-linked dominant trait:
o Appear in both males and females, more frequently in females. o Do not skip generations.
o Affected men will only pass the trait to their daughters, and not their sons.
o Affect woman pass to about half of their sons and half of their daughters.
o A male can only inherit this from his mother.
▪ Y-linked traits:
o Only males are affects.
o If a male if affected, all of his offspring should be affected.
Chromosome structure cont.
o Less condensed
o On chromosome arms
o Unique sequences
o Many genes
o Transcribed often
o Crossing over is common
o More condensed
o On centromeres and telomeres
o Has repeated sequences
o Few genes
o Not transcribed often
o Crossing over is uncommon
o Five major types: H1, H2A, H2B, H3, and H4.
o Net positive charge that attracts to DNA’s net negative charge. ▪ Nucleosome:
o DNA wrapped around an octamer – eight histone proteins. ▪ 2 copies of H2A and H2B and only copy of H3 and H4.
Sex determination systems:
▪ XX-XO: Females XX, Males X
▪ XX-XY: Females XX, Males XY
▪ ZZ-ZW: Females ZW, Males ZZ.
Types of dominance and other terms:
▪ Complete dominance: the heterozygote displays the dominant trait. ▪ Incomplete dominance: the heterozygotes displays a blend of the two traits. o Ex: a heterozygote of red and white parent flowers would produce a pink flower.
▪ Codominance: the heterozygote displays both traits.
o Ex: a heterozygote of red and white parent flowers would produce a flower that has red and white areas on it.
▪ Incomplete penetrance: the genotype doesn’t produce the expected phenotype.
o Ex: polydactyly is a condition of having extra finger or toes. It is often seen that people possess the allele for the condition but have a normal number of fingers and toes.
o Penetrance: the percentage of people that have the genotype and express the phenotype.
▪ Lethal alleles:
o Cause death at early stages of development
o So, the genotype is often never seen.
▪ Multiple alleles:
o When more than two alleles are present at a certain locus. o This increases the number of possible genotypes and phenotypes. ▪ Epistasis:
o When a gene masks the expression of another gene.
▪ Do not assort independently.
▪ This means that you will not see an even number of progenies. ▪ They will segregate together because they are located on the same chromosome but crossing over will cause a recombination of genes. ▪ The parental genes will always be seen in a higher number in the progeny, and the others are recombinant genes.
▪ You can calculate the recombination frequency by summing the number of recombinant progenies, dividing by the total number of progenies produced, and multiplying by 100%. This number will be half the frequency of crossing over and it can only go up to 50%.
▪ One percent of recombination constitutes one map unit. This is used to determine the distances between linked genes.