LINKAGE 2 THINGS TO COVER Process of meiosis in detail Process of crossing over & recombination Genetic variety through movement of chromosomes Mendels 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 chromati d centromere J WERBA IB BIOLOGY
non-sister 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 CHROMOSOMES IN MEIOSIS 10.1.1 , 10.1.2 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 CHROMOSOMES IN MEIOSIS 10.1.1 , 10.1.2 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 (2n).
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 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 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 10.1.3 GENETIC VARIATION 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 10.1.4 INDEPENDENT ASSORTMENT Command term = STATE Mendels 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 10.1.5 INDEPENDENT ASSORTMENT 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 its just lucky that he chose two unlinked traits. J WERBA IB BIOLOGY 25 Sample questions Q1. The diagram below shows chromosomes during meiosis.
How many chromosomes and chiasmata are visible? J WERBA IB BIOLOGY 26 Sample questions Q2. Which processes result in recombination? J WERBA IB BIOLOGY 27 Sample questions Q3. Outline how meiotic division results in almost an infinite
genetic variation in the gametes produced.  Q4. State how chromosome number can increase in human beings.  J WERBA IB BIOLOGY 28 Sample questions A1. C A2. A A3.
crossing over / chiasmata; shuffles alleles; random orientation of chromosomes; at metaphase I: at metaphase II; 2 max A4. non disjunction J WERBA IB BIOLOGY 29
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