Sexual Reproduction: Meiosis

What is Sexual Reproduction? Combines genetic material from two parents to produce a new individual Creates an increase in variation among the offspring Populations can adapt to changing environments The world is more enjoyable with variation Costly – it takes twice as many individuals to produce an offspring

Homologous Chromosomes Genetic material (DNA) is found in chromosomes Each chromosome has a “twin” that: contains the same genes codes for the same traits has the same shape is the same size Has the same location of the centromere Has the same banding pattern when stained One chromosome came from the father, the other came from the mother These twins are called homologous.

Homologous Chromosomes Cells that have two of each chromosome are called Diploid (2n). This is the normal condition for most cells. Cells that have only one of each chromosome are called Haploid (n). This is half of the usual amount of DNA. Homologous Chromosomes

N = 1 – 2 9 3 6 7 10 631 13 23 24 39

Homologous Chromosomes Humans have 23 pairs of chromosomes – 22 pairs of autosomes and one pair of sex chromosomes In females they are all homologous In males, the autosomes are homologous, but the sex chromosomes are not Karyotype – arranges chromosomes in homologous pairs

Special Considerations: 1. If a sperm containing 46 chromosomes fused with an egg containing 46 chromosomes, how many chromosomes would the resulting fertilized egg contain? 2. In order to produce a fertilized egg with the appropriate number of chromosomes (46), how many chromosomes should each sperm and egg have?

Meiosis Purpose: Divide the nucleus to reduce the number of chromosomes in half Ensure that each new cell has a complete set of chromosomes Create gametes Also called reduction division

Human Life Cycle Most of our cells are diploid (2n) Reproductive cells called gametes are haploid (n) They are produced in gonads (ovary and testes) from diploid gametocytes Gametes

Two Stages of Meiosis Meiosis I and Meiosis II A cell normally has two sets of chromosomes (2n) In meiosis I the homologous chromosomes are separated In meiosis II the sister chromatids are separated Four cells are produced, each with one set of chromosomes (n).

Meiosis I The phases of meiosis I are similar to mitosis: Prophase I, Metaphase I, Anaphase I, Telophase I The formation of tetrads is called synapsis Dyads are sister chromatids attached by the centromere

Prophase 1 chromosomes condense homologs pair up forming tetrads (four chromatids are lined up) Nuclear envelope fragments Crossing-over can occur in this stage.

What is Crossing Over? During synapsis of prophase I: Homologous chromosomes tangle and trade parts of their DNA Variation increases Genes that are usually linked (on the same chromosome) can become separated

Metaphase 1 Homologous chromosomes line up at the equatorial plate. Meiosis 1 Metaphase 1 Homologous chromosomes line up at the equatorial plate.

Meiosis 1 Anaphase 1 Homologous pairs separate Sister chromatids remain together.

Meiosis 1 Telophase 1 Two daughter cells are formed Each daughter contains only one chromosome of the homologous pair (haploid).

Prophase II, Metaphase II, Anaphase II, Telophase II Meiosis II After a phase called interkinesis (similar to G2), both cells formed in meiosis I go through phases again Prophase II, Metaphase II, Anaphase II, Telophase II Interkinesis

Meiosis 2 Prophase II chromosomes condense, nuclear envelope fragments

Meiosis 2 Metaphase II Chromosomes align at the equatorial plate.

Meiosis 2 Anaphase II Sister chromatids separate

Meiosis 2 Telophase II Cell division is complete. Four haploid daughter cells are produced.

Meiosis I in an Animal Cell DNA Replication Homologous chromosomes separate, pulled to opposite poles by centromeric spindle fibers. Anaphase I Homologous chromosomes pair during synapsis. Prophase I Meiosis I 2n = 4 Daughter cells have one chromosome from each homologous pair. Telophase I Homologous pairs align at the metaphase plate. Metaphase I Chromosomes still consist of two chromatids. Interkinesis n = 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Meiosis II in an animal Meosis II Prophase II Metaphase II Anaphase II Cells have one chromosome from each homologous pair. n = 2 Metaphase II Chromosomes align at the metaphase plate. Anaphase II Daughter chromosomes move toward the poles. Telophase II Spindle disappears, nuclei form, and cytokinesis takes place. Daughter Cells Meiosis results in four haploid daughter cells. n = 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Nondisjunction Disjunction is the separation of tetrads or sister chromatids Nondisjunction- no separation occurs

Turner Syndrome XO

Klinefelter Syndrome XXY

Down Syndrome (trisomy 21)

Meiosis vs. Mitosis 2n 2n n n 2n Two cells genetically identical to parent cell Four cells not identical to parent

Two daughter cells per cycle Four daughter cells per cycle Mitosis Meiosis One division Two divisions Two daughter cells per cycle Four daughter cells per cycle Daughter cells genetically identical Daughter cells genetically different Same chromosome number as parents Chromosome number half that of parents Occurs in somatic cells Occurs in gametocytes Throughout life cycle Completed after sexual maturity Used in growth, repair, asexual reproduction Sexual reproduction, new gene combinations