Unit 2- Genetics

Meiosis

Fertilization

Zygote46 chromosomes

Egg____ chromosomes

Sperm_____ chromosomes

X

Which is diploid ?Which is haploid ?

Sex cells

Egg23 chromosomes

23 chromosomes

How can a diploid cell become haploid cellsAnswer: by the process of meiosis

Meiosis: process of cell division in which a diploid cell produces four haploid daughter cells; each with half the number of chromosomes found in the parent cell

Recall where DNA is within the cell

Interphase

• DNA replicates/duplicates (S phase)

• In the form of chromatin

• Not easily visible

• Similar to interphase in Mitosis

Before any cell division, DNA must be duplicatedduplicated

• Were going to use a simpler cell in which 2n=4

• This cell have 4 unduplicated chromosomes right before meiosis occurs

Thus, this cell has 2 pairs of homologous chromosomes

• We are now ready for meiosis with Playdoh

What you need

• 3 different colored playdoh– One color for paternal chromosome– One color for maternal chromosome– One color for centromere

• Paper or surface of whiteboard• Pen/marker to draw other details such as cell

membrane, spindle fibers, centrioles etc. • Handout• Group of 3

2 parts of meiosis

• Meiosis 1: reduction division

• Meiosis 2: Gametes are produced

• Unlike mitosis, meiosis does not occur in somatic cells of our body but rather in reproductive organs.

Prophase 1Tetrad: the

homologous pair made of 4 chromatids.

• Chiasmata: region where crossing-over occurs.

• Tetrad: the homologous pair made of 4 chromatids.

• Chiasmata: region where crossing-over occurs.

Prophase 1• Chromosomes condense and become

visible; each with 2 identical sister chromatids.

• Nuclear membrane dissolves; centrioles move to opposite cell poles; spindle fibers form

• Homologous chromosomes pair up and form a tetrad

• Crossing-over occuring between homologous chromosomes produce exchange genetic information

Your cell should look like this now

Metaphase 1

• Homologous chromosomes line up in PAIRS in the middle of the cell

• Again: Homologous chromosomes do not line up in a single file; they line up in pairs in metaphase 1

Anaphase 1

• Homologous chromosomes (NOT individual sister chromatids) separate and move to opposite end of cell.

Telophase 1

• Homologues reach opposite sides of the cell. • A nuclear membrane reforms forming 2 new nuclei• Chromosomes relax

• Two haploid cells (half of the original amount in each cell that is, 2 each)

• Original cell (2n= 4 chromosomes)

http://botit.botany.wisc.edu/images/130/Meiosis/Lilium_microsporogenesis/Meiosis_1.php

Meiosis 2: similar to mitosisProphase 2

•Chromatin coil again•Spindle fibers attach to chromosomes•THERE IS NO DUPLICATION OF CHROMOSOME AT THIS STEP•No Crossing over either

Metaphase 2

• Chromosomes line up in single file RANDOMLY in the middle of each cell

Anaphase 2

• Centromeres split pulling sister chromatids to opposite end of each cell

Telophase 2

• Four nuclei form around chromosomes• Spindle fibers dissolve• Cells divide

Final result of meiosis

4 haploid daughter cells

n = 2

Initial parent cell2n=4

RECAP MEIOSIS https://www.youtube.com/watch?v=vA8aMpHwYh0

Gametogenesis= the formation of gametes

involves

Spermatogenesis in maleand oogenesis in female

No DNA duplication occur here

Chromosome Crossing over

• Only one chiasma is illustrated but many occur per pair

(Plural: chiasmata)

22 = 4 possible combinations to forms a gamete

Some new words• Gamete: a sex cell; includes sperm cells

and egg cells• Zygote: a cell produced by the fusion of

two gametes• Fertilization: the formation of a zygote by

fusing two gametes• Haploid: a cell containing half the usual

number of chromosomes (n)• Diploid: a cell containing two copies of

each chromosome (2n)