Chapter 10 Meiosis

Textbook pages 270 - 276

Reproduction

• Reproduction - process of producing offspring.

• Some offspring are produced by two parents.• Some offspring are produced by just one

parent.• Whether an organism is identical or similar to

its parent is determined by the way the organism reproduces.

Asexual Reproduction

• Production of identical offspring:– Mitosis– Binary Fission– Budding

Today were going to talk about sexual reproduction at the cellular level.

Sexual Reproduction• Practiced by most eukaryotic organisms• In sexual reproduction, two parents give

genetic material to produce offspring that are genetically different from their parents.

• Each parent produces gametes (reproductive cell); sperm and eggs.

• A gamete from one parent fuses with a gamete from the other parent to form a zygote.

• This process is called fertilization.

Germ Cells & Somatic Cells

1. Germ cells (gametes) are cells that are specialized for sexual reproduction (sperm & egg).

2. Somatic cells are all other body cells. They DO NOT participate in sexual reproduction.

All cells contain chromosomes. There are two types of chromosomes found in the nucleus of cells:

1. autosomes2. sex chromosomes

Your body cells contain 23 pairs of homologous chromosomes:

• 23 chromosomes from your mom, and

• 23 chromosomes from your dad

CHROMOSOMES:

NEW Term! • Homologous chromosomes are

chromosomes that make up a pair; one from each parent– Are the same length– Have the same genes

Your body cells contain 23 pairs of homologous chromosomes:

For each homologous pair, one chromosome comes from mom and one comes from dad.

Chromosome pairs 1-22 are autosomes.

The 23rd pair are the sex chromosomes: X and Y which determine gender:

XX =XY =

femalemale

LOOK AT THE KARYOTYPE BELOW:

Haploid vs. Diploid

• Cells that are diploid (somatic cells) have two sets of chromosomes (2n)

• Cells that are haploid (gametes) have one set of chromosomes (n)

Body Cells

- somatic cells- diploid (2n)

Diploid (2n) cells – have two copies of every chromosome

- half the chromosomes come from each parent

Gametes

- sex cells (sperm and egg)- have DNA that can be passed to offspring- haploid (n)

Haploid (n) cells – have one copy of each chromosome

- gametes have 22 autosomes and 1 sex chromosome

Key Terms • Diploid refers to complete

chromosome sets in a cell (or 2N)– N= the number of chromosomes in a

gamete (sex cell)– N chromosomes from the female

parent + N chromosomes from the male parent= 2N

– For humans, 2N = 46

Key Terms • Haploid refers to single

chromosomes in a cell (or N)– One-half of the 2N set (the maternal

N or the paternal N)– Gametes (i.e. sperm, egg sex cells) are

haploid cells– For humans, N = 23

Bottom Line

• diploid means there are the full number of chromosomes in cells

• haploid means that there are half the number of chromosomes in cells

MITOSIS AND MEIOSIS

Mitosis and meiosis are types of nuclear division that make different types of cells.

Mitosis- occurs in somatic cells- makes more diploid cells

Meiosis- occurs in sex cells- Makes 4 haploid cells from diploid cells- produces gametes

Make a Venn diagram to compare andContrast Mitosis and Meiosis!

MEIOSIS10-1

Making gametes…

http://waynesword.palomar.edu/lmexer2a.htm

Cells go through two rounds of division in meiosis:

Meiosis reduces the chromosome number and creates genetic diversity.

homologous chromosomes

sisterchromatids

sisterchromatids

• Meiosis I and Meiosis II - each round has four phases

• Meiosis I – pairs of homologous chromosomes separate

• Meiosis II – sister chromatids divide

Meiosis Interphase Occurs….

• Replication of DNA• Chromatin condenses

Sounds familiar right???

Meiosis I• Occurs after DNA has been replicated in interphase

• Divides homologous chromosomes in four phases: prophase I metaphase I anaphase I telophase I

• After cytokinesis, two cells are produced with one homologous chromosome in each

PROPHASE I • Homologous

chromosomes pair up• Crossing over occurs• Nuclear envelope

breaks down• Spindles form

What is Crossing Over?• Chromosomal segments (genetic information)

are exchanged between a pair of homologous chromosomes

Crossing Over Creates Genetic diversity

Which set of chromatids illustrates the result of a single crossover of the homologous chromosomes?

METAPHASE I

• Spindle fibers attach to the centromeres

• Homologous Chromosomes Line up at the equator

• chromosome pairs line up randomly in cell

ANAPHASE I

• Homologous chromosomes separate and move to the opposite poles of the cell

TELOPHASE I

• The cell divides• The result is two. 2

Haploid daughter cells with exchanged genetic information

• The daughter cells are not genetically identical

**During Telophase I Cytokinesis occurs at the same time!!**

Review Meiosis I

Interphase I Prophase I Metaphase I Anaphase I

.

Telophase I

Meiosis II

• Meiosis II divides sister chromatids in four phases: prophase II metaphase II anaphase II telophase II

• After cytokinesis, four new cells are produced, each with half the chromosome number (haploid)

• Nucleus breaks up• Spindles form and

attach to centromere

• Sister chromatids randomly align at middle of cell

• Sister chromatids separate

• Chromosomes move to opposite poles of cell

•

• Cells divide• 4 haploid

daughter cells result

• Each cell is genetically different

Review Meiosis II

Prophase II Metaphase II Anaphase II Telophase II.

Meiosis Animation

• During meiosis, the egg gets most of the contents• The other cells form polar bodies

• Eggs contribute DNA, cytoplasm and organelles to an embryo

• Sperm become streamlined and mobile with the addition of flagella

• Sperm primarily contribute DNA to an embryo

Haploid cells develop into mature gametes:

Gametogenesis - the production of gametes - differs between females and males

Sources of Variations and Differences1. Crossing over

2. Random assortment of chromosomes

3. Fertilization

• occurs during prophase I• homologous chromosomes swap pieces• this chromosome swapping creates variations

• chromosome pairs line up randomly in cell• when the pairs separate, this creates cells with genetic variations

• which sperm gets to the egg first?? • it is random and can create variations in offspring

MitosisMeiosis

Warm-up

• What is made during Mitosis???

• What is made during Meiosis??

• What are the phases of Meiosis?

Videos that show side by side view

• http://highered.mcgraw-hill.com• http://www.pbs.org/wgbh/nova/miracle/divi_

flash.html

WHY MEIOSIS?• MITOSIS – RESULTS IN GENETICALLY

IDENTICAL OFFSPRING – INCLUDING THE # CHROMOSOMES

• WHAT WOULD HAPPEN IF THE EGG AND SPERM HAD THE SAME # OF CHROMOSOMES AS THE BODY CELLS?

EGG = 46 CHROMOSOMES SPERM = 46 CHROM.

ZYGOTE = 46 + 46 = 92 CHROMOSOMES = NOT HUMAN

Mitosis Animation

Meiosis Animation