Download File - Ms. Richards IB Biology HL

Topic 10.1 Meiosis
Essential idea: Meiosis leads to independent assortment of
chromosomes and unique composition of alleles in daughter cells.
10.1 Meiosis
Nature of science: 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. (1.8)
Understandings
• Chromosomes replicate in interphase before meiosis
• Crossing over is the exchange of DNA material between non-sister homologous
chromatids
• Crossing over produces new combinations of alleles on the chromosomes of the haploid
cells
• Chiasmata formation between non-sister chromatids can result in an exchange of alleles
• Homologous chromosomes separate in meiosis I
• Sister chromatids separate in meiosis II
• Independent assortment of genes is due to the random orientation of pairs of
homologous chromosomes in meiosis I
10.1 Meiosis
Applications and skills
• Skill: Drawing diagrams to show chiasmata formed by crossing over
Review of Definitions
1. Somatic cells: non sex cells
2. Sex cells (gametes): egg (ova), sperm
3. Autosome: chromosome that is not a sex chromosome (does not
determine gender)
4. Sex chromosomes: dissimilar chromosomes that determine an
individual’s sex. In humans: X, Y
5. Homologous chromosomes: pairs of chromosomes that have the
same size, centromere position, staining pattern, location of genes
Review of Definitions
6. Fertilization: the union of two gametes to form a zygote
7. Zygote: a diploid cell that results from the union of 2 haploid
gametes
8. Meiosis: special type of cell division that produces haploid cells
and compensates for the doubling of chromosome number that
occurs at fertilization
Key differences between meiosis and mitosis
1. Meiosis is a reduction division: gametes have ½ number of
chromosomes as parent cell
2. Meiosis creates genetic variation: 4 daughter cells genetically
different from parent cell and from each other
3. Meiosis is 2 successive nuclear divisions
Stages of Meiotic Cell Division
• Interphase I: precedes meiosis
Chromosomes replicate
Each duplicated chromosome consists of 2
identical sister chromatids attached at their
centromere
Centriole pairs in animal cells also replicate into
two pairs
Meiosis I
Prophase I
Segregates the 2 chromosomes of each homologous pair and
reduces the chromosome number by one-half
• Prophase I: 90% of the time required for meiosis.
Longer and more complex than prophase of mitosis.
Chromosomes condense- start to coil up and become
shorter and thicker
Synapsis occurs- homologous chromosomes come together
as pairs. Uses the synaptenemal complex to accomplish this
Prophase I continued
• Each chromosome has 4 chromatids, so that each homologous pair in
synapsis appears as a complex of 4 chromatids of a tetrad
• In each tetrad, sister chromatids of the same chromosome are
attached at their centromeres
• Nonsister chromatids are linked by X-linked chiasmata, sites where
homologous strand exchange or crossing over occurs
• Chromosomes thicken further and detach from the nuclear envelope.
• Centriole pairs move apart, toward poles, and spindle microtubules
form between them
• Nuclear envelope and nucleoli disperse
Chromosome Tetrad
Metaphase I
Metaphase I: tetrads (bivalents) are aligned on the
metaphase plate, having moved there during this stage
Chromosomes continue to shorten and thicken
Spindle microtubules attach to the kinetochore region
of the centromeres
Bivalents line up on the equator so that centromeres of
homologues point towards opposite poles
Chiasmata slide towards the ends of the chromosomes
causing the shapes of the bivalents to change
At the end, chromosomes start to move
Metaphase 1: 2n = 4
Anaphase I
Anaphase I: homologues separate and are moved towards
the poles by the spindle apparatus. (Bivalents separate).
This halves the chromosome number
Each chromosome consists of 2 chromatids (sister
chromatids remain attached at the centromeres)
Because of crossing over, the 2 chromatids are not
identical
At the end, chromosomes reach the poles
Telophase I and cytokinesis
Telophase I and cytokinesis: spindle apparatus continues to
separate homologous chromosome pairs until the
chromosomes reach the poles
Each pole has a haploid set of chromosomes composed
of 2 sister chromatids
Nuclear membranes form around the groups of
chromosomes at each pole
Chromosomes uncoil partially
Cytokinesis occurs simultaneously forming 2 daughter
cells. Cleavage furrows form in animal cells and cell
plates form in plant cells
Meiosis I: reduction division – separation of
homologous chromosomes
Meiosis II: Separation of sister chromatids
Interphase II
Interphase II: May not really exist per se
Two cells either enter a brief period of interphase or immediately
proceed to the second division of meiosis
DNA is NOT replicated
Prophase II
Prophase II: chromosomes become shorter and thicken again by
coiling
Centrioles move to poles (animal cells)
Nuclear membrane breaks down
Spindle apparatus forms and chromosomes move
towards the metaphase II plate
Metaphase II
Metaphase II: chromosomes align singly on the metaphase
plate.
Spindle microtubules attach to the centromeres
Chromosomes line up on equator
Centromeres divide
Kinetochores of sister chromatids point towards
opposite poles
Meiosis II continued
Anaphase II: sister chromatids separate and move toward opposite
poles of the cell
Telophase II and Cytokinesis: nuclei form at opposite poles of the cell
Nuclear membranes reform around the groups of chromatids at
each pole
Cytokinesis produces 4 cells total
Chromosomes uncoil, nucleoli appear
In most organisms, develop into gametes
Meiosis and Fertilization
Primary sources of genetic variation in sexually reproducing organisms
Sexual reproduction provides genetic variation by:
1. Independent Assortment
2. Crossing over during Prophase I
3. Random fertilization by gametes
Independent Assortment of chromosomes
1. Orientation of the homologous pair of chromosomes (one
maternal and one paternal) is RANDOM. 50-50 chance that each
gamete receives maternal or paternal derived chromosome
2. Each homologous pair of chromosomes orients independently of
the other pairs at metaphase I. First meiotic division results in
independent assortment of maternal and paternal chromosomes
Alternative arrangements of 2 homologous
chromosome pairs
Independent Assortment Definitions
• Independent Assortment: The random distribution of maternal and
paternal homologues to the gametes
OR
• Independent Assortment: The presence of an allele of one of two
genes in a gamete has no influence over which allele of another gene
is present in the same gamete
Independent Assortment leads to Genetic
Variation
Process produces 2n possible combinations of maternal and paternal
chromosomes in gametes
If n = 2 then 22 = 4
If n = 23 (human) then 223 or 8,000,000 different possibilities before
crossing over
Crossover
• Definition: the exchange of genetic material between homologues
(homologous portion of 2 non-sister chromatids trade places)
• X-shaped chiasmata are the visible evidence of this process
• Produces chromosomes that contain genes from both parents
• In humans, an average of 3 crossovers/chromosome pair
Crossover
• Most of the time, crossing over can occur without loss of genetic
material because of the precise base to base pairing of homologues
involving the formation of the synaptonemal complex, a protein
structure that brings the chromosomes into close association
Results of crossing over during meiosis
Benefits of Crossing over
1. Chiasmata hold together
homologues during prophase
and metaphase I
2. Allows recombination of linked
genes. Breaks up linkage
groups/parental combinations
Crossing over results in an exchange of alleles
• Parental combinations of linked genes cannot
be broken up without crossing over
• Crossing over occurs between non-sister
chromatids of a homologous pair in prophase
1 between the loci of the 2 linked genes
• Parentals: abc, ABC
• Crossing over occurs between B and C
• Position of chiasma formed by crossing over
• The 4 chromatids separate into 4 nuclei
produced by meiosis
• Recombinants produced = abC, ABc
Linkage Groups
• Definition: All the genes that have their loci on the same
chromosome type form a linkage group
Example of Gene Linkage
Random Fertilization: Result of sexual
reproduction
• In humans an egg cell with 1 of 8,000,000 different possibilities will
be fertilized by a sperm cell that is also 1 of 8,000,000 possibilities,
resulting in a zygote that can have
• 1/64,000,000,000,000 possible diploid combinations
S 10.1.1 Drawing diagrams to show chiasmata
formed by crossing over
• A chiasma is an X-shaped knot-link structure that forms where
crossover has occurred
• Draw two homologous chromosomes using two different colors- one
from mom and one from dad- close to each other
S 10.1.1 Drawing diagrams to show chiasmata
formed by crossing over
• Since the position of crossover is random you can draw it anywhere
(and you can draw more than one)
• The chromosomes will break where they will cross over
• Draw an X-shaped chiasma
S 10.1.1 Drawing diagrams to show chiasmata
formed by crossing over
• Separate and draw the newly crossed over homologous
chromosomes