MEIOSIS AHL Topic 10 1 IB Biology Miss

MEIOSIS AHL Topic 10. 1 IB Biology Miss Werba

AHL TOPIC 10 – GENETICS 10. 1 MEIOSIS 10. 2 DIHYBRID CROSSES & GENE LINKAGE 10. 3 POLYGENIC INHERITANCE J WERBA – IB BIOLOGY 2

THINGS TO COVER �Process of meiosis – in detail �Process of crossing over & recombination �Genetic variety through movement of chromosomes �Mendel’s Law of Independent Assortment J WERBA – IB BIOLOGY 3

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE �Reminder: Homologous chromosomes have the same sequence of genes in the same loci, but slightly different alleles J WERBA – IB BIOLOGY 4

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE � Homologous chromosomes replicate during the synthesis phase (S-phase) of interphase � Yields two pairs of sister chromatids, each joined at a centromere sister chromatids chromatid centromere J WERBA – IB BIOLOGY non-sister chromatids 5

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE � The homologous pair of chromosomes line up side by side during prophase I. � This is called synapsis. J WERBA – IB BIOLOGY 6

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE � The homologous pair of chromosomes line up side by side during prophase I. � This is called synapsis. � The pair formed is called a bivalent. J WERBA – IB BIOLOGY bivalent 7

10. 1. 1, 10. 1. 2 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE � Crossing over may occur between non-sister chromatids during prophase I. � Crossing over can lead to recombination of alleles. � The crossover points are called chiasmata. chiasma J WERBA – IB BIOLOGY 8

10. 1. 1, 10. 1. 2 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE J WERBA – IB BIOLOGY 9

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE � Meiosis reduces the chromosome number in each nucleus. � It forms haploid gametes (n) from diploid somatic cells (2 n). J WERBA – IB BIOLOGY 10

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Interphase � DNA replication occurs � Sister chromatids are formed � Growth and other preparation events also occur J WERBA – IB BIOLOGY 11

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Prophase I � Chromosomes condense � Nuclear membrane disappears � Nucleolus becomes invisible � Spindle formation � Synapsis: ◦ homologous chromosomes move side by side ◦ form bivalents ◦ crossing over may occur(forming chiasmata) results in new allele combinations , thus increasing variation in gametes J WERBA – IB BIOLOGY 12

CROSSING OVER 10. 1. 1 10. 1. 2 10. 1. 3 Command term = OUTLINE Synapsis Homologous chromsomes pair up, forming a bivalent J WERBA – IB BIOLOGY Crossing over Non-sister chromatids may overlap, forming chiasmata Recombination Alleles are swapped between non-sister chromatids, forming recombinant chromosomes. 13

CROSSING OVER 10. 1. 1 10. 1. 2 10. 1. 3 Command term = OUTLINE Crossing over � Increases variation in gametes. � Can break up genes that are usually inherited together due to their proximity to each other. These are called linked genes. J WERBA – IB BIOLOGY 14

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Metaphase I � Bivalents move to the equator. � The chromosomes orientate themselves randomly. also increases variation in gametes as there are 223 possible combinations! J WERBA – IB BIOLOGY 15

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Anaphase I � The spindle fibres contract � Homologous pairs are separated � One chromosome from each pair goes to each pole � Non-disjunction could occur here (ie. failure to separate) and affect the formation of gametes J WERBA – IB BIOLOGY 16

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Telophase I � Chromosomes arrive at poles � Spindle disappears � New nuclei form � The cytoplasm divides by cytokinesis. � The nuclei are now officially haploid. J WERBA – IB BIOLOGY 17

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Prophase II � New spindle forms � Forms at right angles to the previous spindle � Nuclei break down � No crossing over occurs J WERBA – IB BIOLOGY 18

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Metaphase II � Sister chromatids align at the equator � Spindle fibres attach to the centromeres J WERBA – IB BIOLOGY 19

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Anaphase II � Spindle fibres contract � Centromeres break � Sister chromatids are pulled to opposite poles � Non-disjunction could also occur here J WERBA – IB BIOLOGY 20

10. 1. 1 CHROMOSOMES IN MEIOSIS Command term = DESCRIBE Telophase II � Chromosomes arrive at poles � Spindle disappears � Nuclear membrane reappears � Nucleolus becomes visible � Cytokinesis (cell division) is not part of meiosis but is often considered to be the last stage of telophase II J WERBA – IB BIOLOGY 21

GENETIC VARIATION 10. 1. 3 Command term = EXPLAIN � Genetic variation is increased dramatically as a result of meiosis. � Results from: ◦ Crossing over in prophase I ◦ Random orientation of chromsomes in metaphase I ◦ Random fertilisation during reproduction ◦ Mate selection J WERBA – IB BIOLOGY 23

INDEPENDENT ASSORTMENT 10. 1. 4 Command term = STATE � Mendel’s Law of Independent Assortment: The presence of an allele of one gene in a gamete has no influence over which allele of another gene is present. � Only true for unlinked genes ◦ ie. those far away from each other J WERBA – IB BIOLOGY 24

INDEPENDENT ASSORTMENT 10. 1. 5 Command term = EXPLAIN � Random orientation of the chromosomes during metaphase I allows the genes to become independently assorted into the gametes. � Mendel came up with his ideas before the specifics of meiosis were known (ie. random orientation). � Chromosomes and DNA had not been discovered yet. � He was looking at the traits themselves and it’s just lucky that he chose two unlinked traits. J WERBA – IB BIOLOGY 25

Sample questions Q 1. The diagram below shows chromosomes during meiosis. How many chromosomes and chiasmata are visible? J WERBA – IB BIOLOGY 26

Sample questions Q 2. Which processes result in recombination? J WERBA – IB BIOLOGY 27

Sample questions Q 3. Outline how meiotic division results in almost an infinite genetic variation in the gametes produced. [2] Q 4. State how chromosome number can increase in human beings. [1] J WERBA – IB BIOLOGY 28

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