Meiosis Chapter 11.4 Objectives

Describe how homologous chromosomes are alike and how they differ

Contrast haploid and diploid cells

Summarize the process of meiosis

Compare mitosis & meiosis

Explain how genetic variation occurs in offspring of populations

Your body cells have 23 pairs of chromosomes. That is ___ total chromosomes.– Chromosome pairs # 1-22

are autosomes. (=44 chromosomes)

– Pair # 23 are the Sex chromosomes, X and Y, (determine gender)

2 Groups of Chromosomes: 1. Autosomes

2. Sex chromosomes

The pairs are called Homologous Chromosomes

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

Body cells are diploid*

Half the chromosomes

come from each

parent.

Body cells have 44

autosomes & 2 sex

chromosomes

Haploid (n) cells have one copy of every chromosome (or ½ the normal amount)

–Gametes are haploid*

–Gametes have 22 autosome chromosomes and 1 sex chromosome.

–Total = 23 chromosomes

Diploid (2n) vs. Haploid (n) Cells

•Body Cells

•Contain 2 copies of each chromosome

•Total = 46 chromosomes

(44 autosomes & 2 sex)

• Reproduce through mitosis

•Sex Cells

•Contain 1 copy of each chromosome

•Total = 23 chromosomes

(22 autosomes & 1 sex)

• Reproduce through meiosis

Sexual Reproduction relies on Meiosis

Meiosis – a type of nuclear division that produces four haploid gamete cells (each with ½ the number of chromosomes as the parent cell) (humans 46 chromosomes to 23 chromosomes)

Involves germ cells which divide to form gametes (sex cells) (sperm & eggs)

Still occurs after interphase in the cell cycle

**Remember**Mitosis involves body cells, which are also called

somatic cells

Homologous Chromosomes

Body cells are diploid having two sets of chromosomes

One set comes from each parent

Homologous Chromosomes: each chromosome from the male has a corresponding chromosome from the female

Homologous Chromosomes

Homologous chromosomes pair up in meiosis

Half of the homologous chromosomes come from each parent.

Homologous pairs of chromosomes also known as “TETRADS” have the same structure (genes)

• Are pairs of chromosomes which contain a paternal and maternal copy of the same genes

How do we get genetic variation?

Crossing OverDuring Prophase 1 – when

homologous chromosomes while paired in “tetrads” swap alleles.

This creates genetic recombination.

Quick Check / Review1. Your body has what 2 types of cells?

2. Somatic Cells reproduce through ________.

3. Sex Cells reproduce through __________.

4. Cells in your body have what 2 types of chromosomes?

5. Where do pairs of homologous chromosomes come from?

Phases of MeiosisWhat events occur during each phase of

meiosis?• MEIOSIS IS DIVIDED INTO TWO PARTS…• Involves two cell divisions

MEIOSIS I: • Is preceded by a

replication of chromosomes

• Includes:• Prophase I• Metaphase I• Anaphase I• Telophase I

Meiosis is the process that separates homologous pairs of chromosomes in a diploid cell, forming a haploid gamete

MEIOSIS II:• Chromosomes do not

replicate• Includes:

• Prophase II• Metaphase II• Anaphase II• Telophase II

Meiosis results in 4 haploid gametes– Meiosis occurs in sex cells

– Involves 2 nuclear Divisions*– Meiosis produces 4 gametes* which are

haploid (n) cells.

Meiosis 1

Meiosis 2

4 haploid gametes

Parent Germ Cell46 single

46 – Sister Chromatids

23 23

2323 23 23

Note: Without an Interphase between Meiosis 1 and Meiosis 2, the DNA is not duplicated!

During interphaseHomologous Chromosomes

pair up and then move to opposite ends of the cell

Sister chromatids line up and split to opposite ends of the cell

Prophase 1

Metaphase 1

Anaphase 1

Telophase 1

Prophase 2 & Metaphase 2

Anaphase 2

Telophase 2

Meiosis 1

Meiosis 2

2n

n n

n n n n

Mitosis vs. MeiosisMitosis and meiosis are types of nuclear division

that make different types of cells.

Mitosis makes more diploid (2n) cells

Meiosis occurs in sex cellsMitosis occurs in body cells

Meiosis produces gametes

(haploid (n) cells)

Crash Course - MeiosisMeiosis: Where the Sex Starts - Crash Course

Biology #13 - YouTube

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