Unit 3: Cell Division

Left Side Pg Right Side Pg

Unit Page 48 Table of Contents 49

Flow Map – Cell Cycle 50 C.N. – The Cell Cycle 51

Brace Map – Meiosis 52 C.N. – Meiosis 53

Meiosis

Unit 3: Cell DivisionChapter 11-

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Learning Goals

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.

MeiosisMeiosis

A process of cell division that creates gamete cells, or non-identical cells with half the number of chromosomes The gametes from a male and a

female combine in sexual reproduction to make a new individual

Homologous Chromosomes

Each cell has 22 sets of chromosomes Homologous ChromosomesHomologous Chromosomes =

a pair of chromosomes that have the same genes at the same locations

1 came from mom 1 came from dad

From mom

From dad

Homologous chromosomes

Chromosome NumberDiploid (2n)Diploid (2n): cells that have 2 sets of homologous chromosomes (2n)

Human body cells are diploid (4646 chromosomes or 23 pairs)

Haploid (n): cells that have 1 set of chromosomes (n)

sex cells (gametes) are haploidhuman 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)?

meiosismeiosis

Haploid gamete

Diploid

Haploid gamete

2n

n n

Human Female Karyotype

Human Male Karyotype

Down Syndrome Female

Trisomy

Meiosis OverviewA 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 cellEach new cell is genetically different

Steps of Meiosis

InterphaseDNA 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

Meiosis 1

Four steps: Prophase 1 Metaphase 1 Anaphase 1 Telophase 1

Prophase 1

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

Crossing-overHomologous 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 hairFreckles

Chromosome 16

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

Meiosis II

Four steps: Prophase 2 Metaphase 2 Anaphase 2 Telophase 2

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

Principal of Independent Assortment

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 cross-over in a man’s sperm and one cross-over in a woman’s egg, they have the possibility of producing 4,951,760,200,000,000,000,000,000,000 genetically different children

Gamete FormationGametes = Sex Cells (eggs & sperm)

Spermatogenesis: In males meiosis occurs in the testes produces 4 sperm

23

23

23

23

Spermatogenesis

Oogenisis:In females meiosis occurs in the ovaries

produces only 1 egg and 3 polar bodies

Egg23

Polar Bodies

Oogenesis

Fertilization: the fusion of haploid sex cells

Zygote

46

2323

First cell of a new organism

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

Comparing Mitosis & Meiosis

Mitosis results in the production of 2 genetically identical diploid cells.

Meiosis produces 4 genetically different haploid cells.

Learning Goals

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.