LIFE102 Weeks 11 and 12 Notes
LIFE102 Weeks 11 and 12 Notes Life 102
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This 12 page Class Notes was uploaded by Sydney Dingman on Sunday April 10, 2016. The Class Notes belongs to Life 102 at Colorado State University taught by Erik N Arthun in Winter 2016. Since its upload, it has received 35 views. For similar materials see Attributes of Living Systems in Biology at Colorado State University.
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Date Created: 04/10/16
LIFE102 Week 12 Notes 4/4/16, Chapter 16, The Molecular Basis for Inheritance The Structure of DNA Chargaff’s Rules o The base composition (ATCG) of DNA varies between different species o In any species, the number of A and T bases are equal and the number of G and C bases are equal DNA structure o Double-helix o Modeled by Watson and Crick o Watson and Crick’s model explained Chargaff’s rules (A=T and C=G) o 1 DNA nucleotide Phosphate Deoxyribose N-base o A-T base pair: 2 bonds o G-C base pair: 3 bonds (more stable) Structure of 2 DNA strands o Complementary (A-T & G-C) o Antiparallel (3’/5’ 5’/3’) Average chromosome: o 50 million base pairs (bp) o 5,000 genes o 1 gene: 10,000 bp encodes for 1 protein o Before each cell division, all DNA is replicated The basic principle of DNA replication: o Base pairing to a template strand o Since the two strands of DNA are complementary, each strand acts a template for building a new strand in replication o In DNA replication, the parent molecule unwinds and two new daughter strands are built based on base-pairing rules DNA replication is “Semi-conservative” o DNA strands separate: each strand serves as a template for new strand o New strands are made from nucleotides o Each DNA molecule is ½ old and ½ new Starting points of DNA replication: Origin of Replication (ori) o Bacterial chromosomes: 1 ori o Eukaryotic chromosomes: 100-1000 oris DNA Replication o At the end of each replication bubble is a replication fork: a y shaped region where new DNA strands are elongating o Helicases are enzymes that untwist the double helix at the replication forks o RNA primer primes for replication (primase) o Always starts at the 3’ end of the double helix Priming DNA synthesis o Enzymes called DNA polymerases elongate new DNA at a replication fork o DNA polymerases read the DNA and put down the matching enzyme’ 4/6/16, Chapter 16 cont. The antiparallel structure of the double helix affects replication o DNA polymerase add nucleotides only to the free 3’ end of a growing strand; therefore, a new DNA strand can elongate only in the 5’ to 3’ direction o Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork 2 o To elongate the other new strand, called the lagging strand, DNA polymerase must work in the direction away from the replication fork o The lagging strand is synthesized as a series of segments called Okazaki fragments, which are joined together by DNA ligase o DNA Polymerase 1 will later replace the RNA primer fragments with DNA o DNA Ligase connects DNA fragments Makes the backbone of DNA continuous o SLIDE 23 Proofreading & Repairing DNA o DNA polymerases proofread newly-made DNA replacing any incorrect nucleotides o DNA can be damaged by exposure to harmful chemical or physical agents; it can also undergo spontaneous changes o In nucleotide excision repair, a nuclease cuts out damaged stretches of DNA, which are then replaced Replicating the Ends of DNA molecules o Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes o No way to complete the 5’ ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends Telomeres o Strand ends pose a problem for linear DNA: Gaps remain at the end of the 5’ ends when RNA primers are removed o Telomeres: non-coding repetitive DNA segments o TTAGGG repeat in humans o Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called telomeres o Telomeres postpone the erosion of genes near the ends of DNA molecules o It has been proposed that the shortening of telomeres is connected to aging 3 A chromosome consist of DNA molecule packed together with proteins o Chromatin, a complex of DNA and protein is found in the nucleus of eukaryotic cells o Chromosomes fit into the nucleus through an elaborate, multilevel system of packing 4/6/16, Chapter 17, From Gene to Protein The Overview: The Flow of Genetic Information o The information content of DNA is in the form of specific sequences of nucleotides (ATCG) o The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins o Proteins are the links between genotype and phenotype o Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation Basic Princliples of Transcription and Translation o Transcription is the synthesis of RNA using information in DNA o Transcription produces messenger RNA (mRNA) o RNA is the bridge between genes and the proteins for which they code o Translation is the synthesis of a polypeptide, using information in the mRNA o Ribosomes are the sites of translation 4/8/16, Chapter 17, cont. In a eukaryotic cell, the nuclear envelope separates transcription from translation o Eukaryotic RNA transcripts are modified through RNA processing to yield the finished mRNA o In prokaryotes, translation of mRNA can begin before transcription has finished Can happen at the same time in the same place because DNA is located in the cytosol because it doesn’t have nucleus Can make proteins very quickly 4 In Eukaryotes: o DNA: gene library in nucleus Transcription o Messenger RNA: copy made in nucleus brought to cytosol Translation o Protein: made using mRNA info The Central Dogma o Cells are governed by a cellular chain of command o DNA Replication Transcription Translation Protein The Genetic Code How are the instructions for assembling amino acids into proteins encoded into DNA There are 20 amino acids, but there are only 4 nucleotide bases in DNA o Codons: Triplets of Nucleotides The flow of information from gene to protein is based on a triplet code: A series of non-overlapping, three nucleotide words During translation, the mRNA base triplets, called codons are read in the 5’ to 3’ direction The words of a gene are transcribed into complementary nonoverlapping three-nucleotide words of mRNA (codons) These codons are then translated into a chain of amino acids forming a polypeptide (a protein) Transcription o During transcription, one of the two DNA strands, called the template strand, provides a template for ordering the sequence of complementary nucleotides in an RNA transcript o The template strand is always the same strand for a given gene o Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide Molecular components of transcription 5 o RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides o RNA is complementary to the DNA template strand o RNA synthesis follows the same base pairing rules as DNA, except uracil substitutes for thymine Messenger RNA (mRNA): o Made by RNA polymerase o Made from 5’3’ o Complementary and antiparallel with the template DNA Transcription’s 3 stages: o Initiation RNA polymerase binds to promoter Transcriptions starts o Elongation o Termination: RNA polymerase reaches the terminator: Transcript released Eukaryotic cells modify RNA after transcription o Enzymes in the eukaryotic nucleus modify pre-mRNA (RNA processing) before the genetic messages are dispatched to the cytoplasm o Each end of a pre-mRNA molecule is modified The 5’ end reveices a modified nucleotide 5’ cap Handle for a protein to grab on for transportation The 3’ end gets a poly-A tail Prevents RNA molecule from being degraded over time Split Genes and RNA Splicing o Most eukaryotic genes and their RNA transcripts have long noncoding stretching of nucleotides that lie between coding regions o These are noncoding regions are called intervening sequences, or introns 6 o The other regions are called exons becaue they eventually expressed, usually translated into amino acid sequences o RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence Translation o Genetic information flows from mRNA to protein through the process of translation (in power point 3 times) o LOOK AT SLIDE 22 o A cell translates an mRNA message into protein with the help of transfer RNA (tRNA) o tRNAs transfer amino acids to the growing polypeptide in a ribosome Genetic code o The genetic code is redundant (more than one codon may specify a particular amino acid) o Codons must be read in the correct reading frame in order for the right polypeptide to be produced o 7 LIFE 102, Week 11 3/28/16, Chapter 14 cont. Environment affecting genetics can be something like tanning of skin color Pedigree Analysis o A pedigree is a family tree that describes the interrelationships of parents and children across generations o Inheritance patters of particular traits can be traced and described using pedigrees o Pedigrees help us calculate the probability that a future child will have a particular genotype and phenotype Recessively Inherited Disorders o Many genetic disorders are inherited in a recessive manner The Behavior of Recessive Alleles o Recessively inherited disorders show up only in individuals homozygous for the allele o Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents 3/28/16, Chapter 15, The chromosomal Basis of Inheritance Mendelian inheritance has its physical basis in the behavior of chromosomes o Mitosis & Meiosis were first described in the late 1800s o The chromosome theory of inheritance states: Mendelian genes have specific loci (positions) on chromosomes Chromosomes undergo segregation and independent assortment o The behavior of chromosomes during meiosis can account for Mendel’s law of segregation and independent assortment The Chromosomal Basis of Mendel’s Laws o The behavior of nonhomologous chromosomes during meiosis accounts for the independent assortment of the alleles for 2 or more genes on different chromosomes o Starting with 2 true-breeding pea plants, let’s follow 2 genes through the F1 and F2 generations o The two genes specify seed color and seed shape These 2 genes are on different chromosomes o The behavior of chromosomes during meiosis in the F1 generation and subsequent random fertilization gave rise to the F2 phenotypic ratio observed by Mendel Morgan’s Experimental Evidence o Evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan o Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors o Morgan noted wild type (normal) phenotypes that were common in the fly populations o Traits alternative to the wild type are called mutant phenotypes Correlating Behavior of a gene’s alleles with Behavior of a Chromosome Pair o Morgan mated male flies with white eyes with female flies with red eyes o The F1 generation had red eyes o F2 generation showed the 3:1 ratio but only males had white eyes o Morgan determined that the white-eyed mutant allele must be located on the X chromosome Supported the chromosome theory of inheritance Sex-linked genes exhibit unique patterns of inheritance o In humans and some other animals, there is a chromosomal basis of sex determination o There are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome Females XX 2 Males XY o Each Ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome Exceptional inheritance patterns… o X-linked/sex-linked inheritance: different for female and male o X-linked genes: on X chromosome o X & Y: sex chromosomes o X chromosomes: contains lots of genes o Y chromosomes: contain few genes o Men only need one recessive gene on their X instead of having two like women X linked genes o Follow specific patterns of inheritance o For a recessive X-linked trait to be expressed: A female needs two copies of the allele (homozygous) A male only needs one cope of the allele (hemizygous, don’t need to know name) o X linked recessive disorders are much more common in males than in females X Inactivation in Female Mammals o In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development The inactive X condenses into a Barr body o If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Half of her cells will express one allele, while the others will express the alternate allele Mottled coloration, tortoiseshell cats 3/30/16, Chapter 15 cont. Linked genes tend to be inherited together because they are located near eachother on the same chromosome o Each chromosome has hundreds or thousands of genes (except the y) 3 o Genes located on the same chromosome tend to be inherited together are called linked genes o Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters Morgan crossed flies that differed in traits of body color and wing size He found that they must be inherited in specific combinations These genes do not assort independently and are on the same chromosome Exceptional inheritance patterns… o Genes on the same chromosome do not always segregate together o Cause was crossing over Inheritance of Linked Genes o Crossing over-separates linked genes: recombination Chromosomal basis for recombination of linked genes o Because crossing-over between the b and vg loci occurs in some, but not all, egg-producing cells, more eggs with parental-type chromosomes than with recombinant ones are produced in the mating females Calculating Recombinant Frequency o 2 genes further apart on chromosome More change of crossing over Higher recombinant frequency Genetic Alterations o Physical & Chemical disturbances o Mistakes in cell replication o Daughter cells get too much/too little DNA o Possible Mistakes One chromosome too many/few (2n+1, 2n-1): Aneuploidy Extra copies of entire genome ( 3n, 4n, 6n): Polyploidy Meiotic Nondisjunction 4 o In nondisjunction pairs of homologous chromosomes do not separate normally during meiosis o As a result, one gamete receives two of the same type of chromosome and another gamete receives no copy Aneuploidy: abnormal number of a particular chromosome o Down syndrome: 2n=47 Trisomy of chromosome 21: 3 copies of chromosome 21 Human disorders due to chromosomal alterations o Alterations of chromosome number and structure are associated with some serious disorders o Some types of aneuploidy appear to upset the genetic balance lss than others, resulting in individuals surviving to birth and beyond o Surviving individuals have a set of symptoms, or a syndrome, characteristic of the type of aneuploidy Down Syndrome o Down syndrome is a aneuploidy condition that results from three copies of chromosome 21 o It affects about one of every 700 children born in the United States o The frequency of Down syndrome increases with age of the mother, a correlation that has not been explained Polyploidy: extra copies of entire genome o Cause: DNA duplication without cell division o Polypoidy in animals: uncommon Some amphibians and fish Typically lethal in other animals o Polyploidy in plants: fairly common Not lethal: asexual reproduction Many crop plants (tobacco, bananas, strawberries, wheat) 5
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