MEIOSIS3.3 & 10.1

Meiosis:A reduction division of a diploid nucleus

to form four haploid nuclei.This allows for a sexual life cycle in living

organisms.

Number of Chromosomes

Description of condition

Cell Type

46 Diploid (2N) Typical body (somatic) cell

23 Haploid (N) Gamete,Egg or Sperm cell

Chromosomes

Homologous chromosomes: in a diploid cell, 46 chromosomes are grouped into 23 pairs of chromosomes.

Homologous: similar shape and size, and carry the same genes

Meiosis

Interphase I –All the chromosomes are duplicated and thus

each consists of two identical sister chromatids.

Figure 13.4

Key

Maternal set ofchromosomes (n = 3)

Paternal set ofchromosomes (n = 3)

2n = 6

Two sister chromatidsof one replicatedchromosome

Two nonsisterchromatids ina homologous pair

Pair of homologouschromosomes(one from each set)

Centromere

In Meiosis I:◦Prophase I – Each chromosome pairs with its

corresponding homologous chromosome to form a bivalent (a.k.a. tetrad)

Crossing Over occursduringprophase I, then the chromosomes condense.

Crossing Over

During crossing over there is exchange of DNA material between non-sister homologous chromatids.

This produces newcombinations ofalleles on the chromosomes of thehaploid cells.

This leads to geneticvariation.

Figure 13.11

Prophase Iof meiosis

Nonsisterchromatids

Bivalent

Chiasma,site ofcrossingover

Metaphase I

Metaphase II

Daughtercells

Recombinantchromosomes

ChiasmataA chiasma is an X-shaped knot-like

structure that forms where crossing over has occurred. ◦It holds a bivalent together for a while after the

chromosomes condense by supercoiling.

Centrosomes(with centriole pairs)

Sisterchromatids

Chiasmata

Spindle

Tetrad

Nuclearenvelope

Chromatin

Centromere(with kinetochore)

Microtubuleattached tokinetochore

Bivalents line up

Metaphaseplate

Homologouschromosomesseparate

Sister chromatidsremain attached

Pairs of homologouschromosomes split up

Chromosomes duplicateHomologous chromosomes

(red and blue) pair and exchangesegments; 2n = 6 in this example

INTERPHASE MEIOSIS I: Separates homologous chromosomes

PROPHASE I METAPHASE I ANAPHASE I

Interphase and meiosis I

Figure 13.8

After finishing Meiosis I, our results are two daughter cells with a haploid number of duplicated chromosomes.

Meiosis II

TELOPHASE I ANDCYTOKINESIS

PROPHASE II METAPHASE II ANAPHASE II TELOPHASE II ANDCYTOKINESIS

MEIOSIS II: Separates sister chromatids

Cleavagefurrow Sister chromatids

separate

Haploid daughter cellsforming

During another round of cell division, the sister chromatids finally separate;four haploid daughter cells result, containing single chromosomes

Two haploid cellsform; chromosomesare still doubleFigure 13.8

Telophase I, cytokinesis, and meiosis II

Meiosis I◦Homologous chromosomes separate◦Reduces the number of chromosomes from diploid to

haploid Meiosis II

◦Sister chromatids separate◦Produces four haploid daughter cells

Genetic Variation

Genetic Variation is increased by:◦Crossing over (during prophase I)◦(Random) Fusion of gametes◦Independent assortment

Sexual Reproduction

Fusion of gametes from different parents promotes genetic variation.◦This allows alleles from two different individuals

to be combined into one new individual.◦The combination of alleles is unlikely ever to

have existed before genetic variation.◦Genetic variation is essential for evolution of a

species.

Independent Assortment of genesOrganization/ orientation of pairs of homologous

chromosomes during metaphase is random.

Figure 13.10

Key

Maternal set ofchromosomesPaternal set ofchromosomes

Possibility 1

Two equally probable arrangements ofchromosomes at

metaphase I

Possibility 2

Metaphase II

Daughtercells

Combination 1 Combination 2 Combination 3 Combination 4

Karyotyping

Non-disjunction: “not coming apart” – when chromosomes fail to separate during Meiosis 1 or 2.

Gametes contain two copies or no copies of a particular chromosome.

Offspring have an extra or missing chromosome.

Figure 15.12a, b

Meiosis I

Nondisjunction

Meiosis II

NondisjunctionGametes

n + 1n + 1 n 1 n – 1 n + 1 n –1 n nNumber of chromosomes

Nondisjunction of homologouschromosomes in meiosis I

Nondisjunction of sisterchromatids in meiosis II

(a) (b)

Down’s Syndrome – Trisomy 21◦The person has 3 (instead of 2)

21st chromosomes

Age of parents vs. Down Syndrome

Do the DBQ on pg. 167 – 168: “Parental age and non-dsjunction”

Karyotype: a property of a cell – the number and type of chromosomes present in the nucleus.

Karyogram:picture of chromosomes arranged in pairs,according to their size and structure(banding patterns).

Chromosomal abnormalities

Trisomy 18, Trisomy 13

Turner’s Syndrome – females with only one X

Klinefelter’s Syndrome – males with XXY

Karyotyping is used for pre-natal (before birth) diagnosis of chromosome abnormalities.

Where do we get the cells for doing a karyotype?

1) amniocentesisExtract amniotic fluid,Inside are some of the

baby’s cells

Risks:◦Miscarriage 1 in 200 to

1 in 400◦Accuracy: 99.4%

2) chorionic villus sampling

Tissue sample from the placenta’s projections into the uterus wall

Risks?◦Slightly higher chance of

miscarriage than amniocentesis because it is done earlier in pregnancy.

◦Accuracy: 98%