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Unit Page
Flow Map – Cell Cycle
Brace Map – Meiosis
Pg Right Side
48 Table of Contents
50 C.N. – The Cell Cycle
52 C.N. – Meiosis
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53
Pg
49

Unit 3:
Cell Division
Chapter 11-4

1. Describe the overall process of meiosis.
2. Define homologous chromosomes and explain diploid and haploid.
3. Explain crossing over, its importance, and during which step it occurs.
4. Describe the two types of gamete formation and how fertilization occurs.

A process of cell division that creates gamete cells , or nonidentical cells with half the number of chromosomes

The gametes from a male and a female combine in sexual reproduction to make a new individual

Each cell has 2 sets of chromosomes

Homologous Chromosomes = a pair of chromosomes that have the same genes at the same locations
 1 came from mom
 1 came from dad

Homologous chromosomes
From mom
From dad

Diploid (2n) : cells that have 2 sets of homologous chromosomes (2n)
 Human body cells are diploid ( 46 chromosomes or 23 pairs)
Haploid (n) : cells that have 1 set of chromosomes (n)
 sex cells ( gametes ) are haploid
 human haploid cells (egg & sperm) have 23 chromosomes total

1) A bug has a haploid number n=5. What is the diploid number (2n)?
2) A crocodile has a diploid number 2n=50.
What is the haploid number (n)?
meiosis
Diploid
2n meiosis
Haploid gamete n n
Haploid gamete

Trisomy

A type of cell division specific to sexual reproduction (two parent cells)
Cells (diploid) divide twice resulting in 4 daughter cells (haploid)
Each cell has half the number of chromosomes as the original cell
Each new cell is genetically different

DNA is copied before the start of Meiosis I.
This makes 2 identical sister chromatids

They have the same genes at the same locations.

Homologous chromosomes vs. sister chromatids
From mom
From dad

Four steps:

Prophase 1



Metaphase 1
Anaphase 1
Telophase 1

The homologous pairs of chromosomes come together and line up side by side . (tetrad = 4)

Homologous chromosomes can
Cross-over and exchange a portion of the chromosome
(DNA) at random

Crossing-over is very important in meiosis because it creates genetic variation (new combinations of genes), which makes us all different.

This is why you do not look exactly like your siblings.

Crossing over happens randomly.
However, some genes are linked, or close together on the chromosome, meaning they are more likely to be inherited together.
Red hair
Freckles
Chromosome 16

After crossing-over, the homologous chromosomes separate & the cells divide

Four steps:




Prophase 2
Metaphase 2
Anaphase 2
Telophase 2

Sister chromatids separate & the cells divide.
Result is 4 genetically different haploid daughter cells

Genes for different traits can segregate (separate) independently during the formation of gametes.
 Independent assortment and crossing-over account for the many genetic variations observed in plants, animals, and other organisms.

Assume there was only one crossover in a man’s sperm and one crossover in a woman’s egg, they have the possibility of producing
4,951,760,200,000,000,000,000,000,000 genetically different children

Gametes = Sex Cells ( eggs & sperm )
 Spermatogenesis:

In males meiosis occurs in the testes
 produces 4 sperm
23
23
23 23

 Oogenisis:

In females meiosis occurs in the ovaries
 produces only 1 egg and 3 polar bodies
Polar Bodies
Egg
23

Fertilization : the fusion of haploid sex cells
23 23
Zygote
46
First cell of a new organism

Only one sperm and one egg come together
The first sperm to reach the egg will be the one to fertilize it.

Mitosis results in the production of 2 genetically identical diploid cells.
Meiosis produces 4 genetically different haploid cells .

1. Describe the overall process of meiosis.
2. Define homologous chromosomes and explain diploid and haploid.
3. Explain crossing over, its importance, and during which step it occurs.
4. Describe the two types of gamete formation and how fertilization occurs.