Genetics3.3 Meiosis10.1 Meiosis (AHL)

Recall Mitosis

•Purpose:•Process:•Results:

Sexual Reproduction

•What is sexual reproduction?•Why is it beneficial?•What kinds of cells are involved?•How are these cells formed?

Diploid vs Haploid

•Diploid nuclei have pairs of homologous chromosomes.

•Haploid nuclei have one chromosome of each pair.

Meiosis•Meiosis is the process by which sex cells,

or gametes, are formed.▫In humans this takes place in the testes and

ovaries•Meiosis involves 2 stages of cell division.•In the end, daughter cells contain half the

chromosome number of the parent cell = haploid.

•Halving of the chromosome number allows a sexual life cycle with fusion of the gametes.

Stages of Meiosis: (Make a table as shown on the

board)

• Meiosis I:▫ Purpose: Reduce chromosome number▫ Process: Prophase I, Metaphase I, Anaphase I,

Telophase I▫ Results: Two Daughter cells each containing half the

number of chromosomes (still in the form of sister chromatids).

• Meiosis II:▫ Purpose: Separate sister chromatids▫ Process: Prophase II, Metaphase II, Anaphase II,

Telophase II▫ Results: Total of Four haploid Daughter cells (which

may become gametes).

Crash Course Biology Video:https://www.youtube.com/watch?v=qCLmR9-YY7o

Meiosis I•Prophase I:

▫Nuclear membrane dissolves▫Centrioles split and move to opposite poles▫Spindle fibers form▫Chromosomes come together in

homologous pairs Each chromosome is composed of a pair of

sister chromatids The whole structure is referred to as a tetrad

because it contains 4 chromatids▫The intertwined chromatids may break and

exchange segments of genetic material = crossing over

Meiosis I

•Metaphase I:▫Homologous chromosomes attach to

spindle fibers▫In pairs, they line up along the equatorial

plate

Meiosis I

•Anaphase I:▫“Reduction division” occurs▫One member of each Homologous pair

moves towards either pole. Each chromosome still contains sister

chromatids

Meiosis I

•Telophase I:▫Membranes form to create 2 new nuclei▫Each of the daughter nuclei contains one

member of each chromosome pair.

▫Telophase is followed by cytokinesis, the division of the cytoplasm.

Meiosis II

•Prophase II:▫Nuclear membrane dissolves▫Spindle fibers form

Meiosis II

•Metaphase II:▫Chromosomes, each with two chromatids,

line up along the equatorial plate.

Meiosis II

•Anaphase II:▫Attachment between the sister chromatids

is broken▫Individual chromatids move to opposite

poles

Meiosis II

•Telophase II:▫Nuclear membranes form▫Form haploid nuclei have been produced

▫Telophase is followed by cytokinesis.

Meiosis

•Complete the worksheets

Meiosis

•Complete the practical activity▫P 110 and 111 in Bio 11 textbook

Bivalent formation and crossing over• The early stages of

meiosis involve pairing of homologous chromosomes and crossing over followed by condensation.

• Bivalent = a pair of homologous chromosomes

• Synapsis = pairing process

• Junction = the area where chromosomes break and rejoin

• Chiasmata = the point of connection

Random Orientation of the bivalents

• Orientation of pairs of homologous chromosomes prior to separation is random.

• Orientation = the pole to which each chromosome is attached depends on the way it is facing.

• For every additional bivalent, the number of possible chromosome combinations in a cell produced by meiosis doubles (2n).▫ Calculate the number of possible

combinations in humans

Non-disjunction• Non-disjunction = when

homologous chromosomes fail to separate in anaphase.

• Both of the chromosomes move to one of the poles.

• The result is a gamete with either an extra chromosome or deficient in a chromosome.

• If this gamete goes on to be fertilized, the result will be an individual with 45 or 47 chromosomes.

• Example: Down Syndrome