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 Over During Prophase 1 – when homologous chromosomes while paired in “tetrads” swap alleles. This creates genetic recombination.

Quick Check / Review 1. 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 Meiosis What 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 Cell 46 single 46 – Sister Chromatids 23 Note: Without an Interphase between Meiosis 1 and Meiosis 2, the DNA is not duplicated! During interphase Homologous 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 nn nnnn

Mitosis vs. Meiosis Mitosis 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 - Meiosis Meiosis: Where the Sex Starts - Crash Course Biology #13 - YouTube Meiosis: Where the Sex Starts - Crash Course Biology #13 - YouTube

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