The history of Life on Earth reflects an unbroken chain of genetic continuity and transmission of genetic information:

Transcription

1 9/26/05 Biology 321 Answers to optional challenge probability questions posed in 9/23/05lecture notes are included at the end of these lecture notes The history of Life on Earth reflects an unbroken chain of genetic continuity and transmission of genetic information: How many years of genetic continuity does this unbroken chain represent? 1

2 The processes of mitosis and meiosis are common to all eukaryotic cells suggesting that these processes evolved in very ancient eukaryotic cells that were the ancestors to all present-day eukaryotic cells How old are these processes then? 2

3 Transmission or propagation of any genetic program requires two basic steps: 1. Copying over step (replication of the DNA molecule) 2. Parcelling out step (getting the duplicated information properly distributed to the progeny) In the nucleus of eukaryotic cells, the parcelling out process occurs in the form of 2 different types of cell division: 3

4 Mitosis is a conservative propagation of genetic information: the daughter cells have the same genetic composition as the mother cell Meiosis is a non-conservative propagation of genetic information: the products of meiosis have a different genetic composition than the mother cell 4

5 Review cell cycle and stages of mitosis label the stages: prophase, metaphase, anaphase, telophase 5

6 Stages of mitosis in the cells of the Indian muntjak, a deer species where 2n=6. This small number of chromosomes allows clear visualization of the condensation (yellow to orange), separation (red to turquoise) and decondensation (green to yellow) of chromosomes throughout mitosis. 6

7 At the end of prophase of mitosis each duplicated chromosome is maximally condensed Each chromosome consists of 2 identical halves known as sister chromatids Each chromatid contains one double-stranded DNA molecule coiled up Sister chromatids are held together at a constriction called the centromere scanning electron micrograph of human chromosomes metaphase chromosome spread Note the size variation and that the centromere is located at different positions in different chromosomes 7

8 Diploid cell n = 2, 2n = 4 homologs G1, S, G2 sister chromatids centromere (= kinetochore + DNA) OR Meiosis I Mitosis Meiosis II 8

9 2n=2 in this example n= number of chromosomes in one copy of the (nuclear) genome 1n = haploid 2n = diploid 3n= triploid 4n = tetraploid the number preceding the n indicates the number of genome copies (or number of copies of each chromosome) Chromosome number (n) and genome size do not necessarily correlate 9

10 Connecting the products of semi-conservative DNA replication with sister chromatids: Each sister chromatid is composed of a single, continuous polymer of double-strand DNA coiled into a compact form:: HOW LONG ARE THESE POLYMERS? The next figure diagrams the connection between semiconservative replication of DNA and the sister chromatids 10

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12 Meiosis is a non-conservative propagation of genetic information: the products of meiosis have a different genetic composition than the mother cell Meiosis I (reductional division) Homologous chromosomes are separated into different cells: each daughter cell contains one duplicated copy of each type of chromosome (the number of chromosomes is cut in half) Meiosis II (equational division) Sister chromatids are separated into different cells (like a mitotic division) 12

13 What events occur during prophase of meiosis that do not occur during prophase of mitotis? 13

14 synapsis: close pairing of homologs during prophase of Meiosis I What is a homolog? 14

15 Homologous chromosomes chromosomes that pair with each other during meiosis chromosomes that possess the same genes at corresponding positions Diploids have two copies or two homologs of each different type of chromosome 15

16 crossing over: a precise breakage and reunion event that occurs between two non-sister chromatids pieces of the DNA strands in the two chromatids are exchanged 16

17 note: we will look at crossing over in detail later in the quarter 17

18 Diploid cell n = 2, 2n = 4 homologs G1, S, G2 sister chromatids centromere (= kinetochore + DNA) OR Meiosis I Mitosis Meiosis II 18

19 Walter Sutton was a Kansas farm boy and in 1902 was the first person to point out that: 19

20 Chromosomes obey Mendel s rules of segregation and independent assortment Interested in the History of genetics? check out this web site: Mendel s principle of segregation: Paired hereditary factors (R and r alleles in this diagram) segregate into different gametes 20

21 MENDEL S PRINCIPLE OF SEGREGATION: During gamete formation the members of each gene pair segregate (that is, separate) into different gamete cells. Segregation of alleles into different gametes is ensured by the pairing of homologs during prophase of the first meiotic division and the separation of homologs into different daughter cells in anaphase of the first meiotic division 21

22 What events in meiosis explain Mendel s principle of independent assortment? 22

23 In metaphase of the first meiotic division, the paired homologs (the bivalents) line up randomly with respect to each other. Meiosis generates gametes with many genetic combinations Diploid cell n = 2, 2n = 4 Independent assortment OR Meiosis I Meiosis II Crossing over Meiosis I Meiosis II 23

24 NOTE that allele symbols are different here from what we used in class G= yellow g= green W = round w=wrinkled yellow round x green wrinkled P: GGWW x ggww gw gamete gamete GW GgWw F1 zygote F1 x F1 G w g W OR G W g w Meiosis I G w g W G W dg Tw Gametes G W male gametes G w g W dg Tw G W female gametes g G w W dg Tw On quiz Friday must indicate location of all alleles for Meiosis I --shorthand in this diagram not acceptable. See also answer sheet--question 7.) [If time: Mendel and independent assortment -- table --genomic location of pea traits] 24

25 State a general rule of thumb to answer this question: What organisms will show Mendelian patterns of inheritance? 25

26 Since meiosis provides the physical basis for Mendel s genetic principles, all organisms that have meiosis as part of their lifecycle will show Mendelian patterns of inheritance Does this include haploid orgnanisms? 26

27 Do mitosis and meiosis account for the propagation of all genetic information represented by Life on Earth? What are the exceptions? 27

28 Prokaryotes: binary fission Eukaryotes: mitochondrial and chloroplast DNA Viruses: parasitize cells Genes not propagated by meiosis will not show Mendelian patterns of inheritance 28

29 What happens during meiosis in an organism that has an odd number of genome copies? The problems with bananas who haven t had sex in decades-- see article on 321 web site 29

30 Answers to challenge problems Problem 1 (3/4) 10 = 0.056=1 in 18 hets will produce only dominant progeny and be miscored as homozygotes for the dominant allele Problem 2 Homozygotes for the dominant allele will breed true and always be scored correctly: 0.33 of F2 with dominant phenotype scored correctly For each of the ten progeny of a heteozygote: 3/4 chance it shows the dominant phenotype 1/4 chance it shows the recessive phenotype (which would reveal the genotype of the parent) (3/4) 10 = 0.056= chance that all ten progeny of a heterozygous plant will show the dominant phenotype = probability that at least one progeny of a heterozygous plant is homozygous recessive = /3 [fraction of F2 with dominant phenotype that are het] [(3/4) 10 ]= scored incorrectly scored as RR = 0.33 (correct) (incorrect) = 0.37 scored as Rr = 0.67 (0.944) = Rr to 0.37 RR = 1.7 to 1 30