MEIOSIS Meiosis leads to independent assortment of chromosomes

MEIOSIS Meiosis leads to independent assortment of chromosomes and unique composition of alleles in daughter cells. AHL Topic 10. 1 IB Biology Miss Werba

TOPIC 10 – GENETICS and EVOLUTION 10. 1 MEIOSIS 10. 3 GENE POOLS and SPECIATION J WERBA – IB BIOLOGY 10. 2 INHERITANCE 2

THINGS TO COVER Statement Guidance U. 1 Chromosomes replicate in interphase before meiosis. U. 2 Crossing over is the exchange of DNA material between non-sister homologous chromatids. U. 3 Crossing over produces new combinations of alleles on the chromosomes of the haploid cells. U. 4 Chiasmata formation between non-sister chromatids can result in an exchange of alleles. U. 5 Homologous chromosomes separate in meiosis I. U. 6 Sister chromatids separate in meiosis II. U. 7 Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I. S. 1 Drawing diagrams to show chiasmata formed by crossing over. Should show sister chromatids still closely aligned, except at the point where crossing over occurred and a chiasma was formed. Making careful observations—careful observation and record keeping turned up anomalous NOS 1. 8 data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies. J WERBA – IB BIOLOGY 3

MEIOSIS http: //www. stolaf. edu/people/giannini/flashanimat/celldivision/meiosis. swf J WERBA – IB BIOLOGY 4

https: //youtu. be/eaf 4 j 19_3 Zg? t=15 s

HOMOLOGOUS CHROMOSOMES � Homologous chromosomes have the same sequence of genes in the same loci, but slightly different alleles J WERBA – IB BIOLOGY 6

HOMOLOGOUS CHROMOSOMES U. 1 � Chromosomes replicate during the synthesis phase (S-phase) of interphase before meiosis. J WERBA – IB BIOLOGY 7

HOMOLOGOUS CHROMOSOMES U. 1 � Replication yields two pairs of sister chromatids, each joined at a centromere sister chromatids chromatid centromere non-sister chromatids J WERBA – IB BIOLOGY 8

HOMOLOGOUS CHROMOSOMES U. 1 � The homologous pair of chromosomes line up side by side during prophase I. � This is called synapsis and the pair formed is called a bivalent J WERBA – IB BIOLOGY 9

CROSSING OVER � Might U. 2 U. 4 occur in Prophase I � Sections of non-sister chromatids may touch (ie. cross over). � This point is called a chiasma (pl. chiasmata). � Sections of the chromosomes may be swapped between the non-sister chromatids. � This produces recombinant chromosomes (ie. with allele combinations unlike either parent chromosome) J WERBA – IB BIOLOGY 10

U. 3 CROSSING OVER � Increases genetic variation through the recombination of linked alleles. J WERBA – IB BIOLOGY 11

U. 3 CROSSING OVER Synapsis Crossing over Recombination Homologous chromsomes pair up, forming a bivalent Non-sister chromatids may overlap, forming chiasmata Alleles are swapped between non-sister chromatids, forming recombinant chromosomes. J WERBA – IB BIOLOGY 12

U. 3 CROSSING OVER Y Y Z Z y y z z y y z Z Y Y z Z recombinants J WERBA – IB BIOLOGY 13

U. 3 CROSSING OVER � Skill: you need to be able to draw diagrams to show chiasmata formed by crossing over ◦ Try to differentiate between the 2 sister chromatids in some way (remember no colours or shading). ◦ Remember that the homologous chromosomes are in synapsis and therefore need to be drawn close together. ◦ Make sure the sister chromatids, centromeres and chiasmata are all labeled. J WERBA – IB BIOLOGY 14

INDEPENDENT ASSORTMENT U. 7 � Independent assortment of genes is due to the random orientation of pairs of homologous chromosomes in meiosis I � Occurs in Metaphase I � Bivalents move to the equator. � The chromosomes orientate themselves randomly. ◦ increases variation in gametes as there are 223 possible combinations! J WERBA – IB BIOLOGY 15

INDEPENDENT ASSORTMENT U. 7 � Only holds true for unlinked genes (ie. genes on different chromosomes) J WERBA – IB BIOLOGY 16

SEPARATION OF CHROMOSOMES U. 5 Non-sister chromatids � Homologous pairs are separated in Anaphase I as the spindle fibres contract. � One chromosome from each pair goes to each pole. � This is a reductive division (ie. 2 n n). � Non-disjunction could occur here (ie. failure to separate) and affect the formation of gametes. J WERBA – IB BIOLOGY 17

SEPARATION OF CHROMOSOMES U. 6 Sister chromatids � The duplicated chromosomes (sister chromatids) separate during Metaphase II. � Again, random orientation contributes to variation in the gametes. J WERBA – IB BIOLOGY 18

MENDEL vs MORGAN NOS 1. 8 Making careful observations Careful observation and record keeping turned up anomalous data that Mendel’s law of independent assortment could not account for. Thomas Hunt Morgan developed the notion of linked genes to account for the anomalies. J WERBA – IB BIOLOGY 19

NOS 1. 8 MENDEL vs MORGAN � Gregor Mendel deduced that: “The presence of an allele of one of the genes in a gamete has no influence over which allele of another gene is present. ” � Independent assortment occurs as a result of the random orientation of chromosomes during Metaphase I. � Mendel made his deduction based on his work with pea plants. It was just lucky that he investigated two traits that were on separate chromosomes (ie. unlinked genes). J WERBA – IB BIOLOGY 20

NOS 1. 8 MENDEL vs MORGAN � Thomas Hunt Morgan investigated traits in fruit flies. � He produced results that could not be explained by Mendel’s work. � The “anomalous” data was repeated and kept appearing in predictable patterns. � Morgan and his colleagues revised Mendel’s ideas: ◦ Discrete pairs of factors (ie. genes) are located on chromosomes ◦ Some traits are sex-linked ◦ Other traits are also sometimes linked J WERBA – IB BIOLOGY 21

https: //www. dnalc. org/view/15005 -Thomas-Hunt-Morgan. html

MEIOSIS Q 1. The diagram below shows: A. B. C. D. Four separate chromosomes A bivalent One pair of sister chromatids Non-disjunction J WERBA – IB BIOLOGY 23

MEIOSIS Q 2. The diagram below shows: A. B. C. D. Two separate chromosomes A bivalent One pair of sister chromatids Crossing over J WERBA – IB BIOLOGY 24

MEIOSIS Q 3. The diagram below shows: A. B. C. D. Two separate chromosomes A bivalent One pair of sister chromatids Homologous chromosomes J WERBA – IB BIOLOGY 25

MEIOSIS Q 4. The diagram below shows chromosomes during meiosis. How many chromosomes and chiasmata are visible? J WERBA – IB BIOLOGY 26

MEIOSIS Q 5. Which processes result in recombination? J WERBA – IB BIOLOGY 27

MEIOSIS Q 6. Outline how meiotic division results in almost an infinite genetic variation in the gametes produced. [2] Q 7. State how chromosome number can increase in human beings. [1] J WERBA – IB BIOLOGY 28

MEIOSIS � A 1. � A 2. � A 3. � A 4. � A 5. � A 6. B C A ◦ crossing over / chiasmata; ◦ shuffles alleles; ◦ random orientation of chromosomes; ◦ at metaphase I: ◦ at metaphase II; 2 max � A 7. non disjunction J WERBA – IB BIOLOGY 29