Sexual Reproduction and Meiosis Chapter 8

Sex Asexual reproduction - individual inherits all its chromosomes from a single parent parthenogenesis - development of an adult from an unfertilized egg

Sexual reproduction - Fusion of two gametes to produce a single zygote. Introduces greater genetic variation, allows genetic recombination. With exception of self-fertilizing organisms (e.g. some plants), zygote has gametes from two different parents.

Reduction Division In sexual reproduction, gametes fuse (fertilization) to produce a zygote. Gamete formation involves a mechanism (meiosis) that reduces the number of chromosomes to half that found in other cells. Adult body cells are diploid (2N) Gamete cells are haploid (1N)

A diploid cell Has two sets of each of its chromosomes In a human has 46 chromosomes (2n = 46) In a cell in which DNA synthesis has occurred All the chromosomes are duplicated and thus each consists of two identical sister chromatids

Unlike somatic cells Gametes, sperm and egg cells are haploid cells, containing only one set of chromosomes = 1n = 23

Sexual Life Cycle

At sexual maturity The ovaries and testes produce haploid gametes by meiosis During fertilization - These gametes, sperm and ovum, fuse, forming a diploid zygote The zygote develops into an adult organism Most cells in the body produced by mitosis. Only gametes are produced by meiosis.

MEIOSIS Meiosis reduces the number of chromosome sets from diploid to haploid Meiosis Takes place in two sets of divisions, meiosis I and meiosis II Cell division results in 4 haploid (1N) cells from a single parent cell N = 23 in humans

The Stages of Meiosis Meiosis I An overview of meiosis Meiosis I Reduces the number of chromosomes from diploid to haploid Meiosis II Produces four haploid daughter cells Interphase Homologous pair of chromosomes in diploid parent cell Chromosomes replicate Homologous pair of replicated chromosomes Sister chromatids Diploid cell with replicated chromosomes 1 2 Homologous separate Haploid cells with replicated chromosomes Sister chromatids Haploid cells with unreplicated chromosomes Meiosis I Meiosis II

Interphase and meiosis I Centrosomes (with centriole pairs) Sister chromatids Chiasmata Spindle Tetrad Nuclear envelope Chromatin Centromere (with kinetochore) Microtubule attached to kinetochore Tertads line up Metaphase plate Homologous chromosomes separate Sister chromatids remain attached Pairs of homologous chromosomes split up Chromosomes duplicate Homologous chromosomes (red and blue) pair and exchange segments; 2n = 6 in this example INTERPHASE MEIOSIS I: Separates homologous chromosomes PROPHASE I METAPHASE I ANAPHASE I

Chiasmata indicate point where the two chromatids are intertwined. Prophase I Homologous chromosomes become closely associated in synapsis, exchange segments via crossing over, and then separate. Chiasmata indicate point where the two chromatids are intertwined. chiasmata

Metaphase I Spindle microtubules attach to kinetochore proteins on the outside of each centromere. Joined pairs of homologues lines up on metaphase plate. orientation of each pair is random

Random orientation of chromosomes in Metaphase Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Random orientation of chromosomes in Metaphase

function as one. Microtubules can attach to only one side of Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Homologues do not pair; kinetochores of sister chromatids remain separate; microtubules attach to both kinetochores on opposite sides of the centromere. Chiasmata hold homologues together. The kinetochores of sister chromatids fuse and function as one. Microtubules can attach to only one side of each centromere. Meiosis I Mitosis Metaphase I Metaphase Chiasmata Microtubules pull the homologous chromosomes apart, but sister chromatids are held together. Microtubules pull sister chromatids apart. Anaphase I Anaphase

Anaphase I Spindle fibers begin to shorten and pull whole centromeres toward poles. Each pole receives a member of each homologous pair. complete set of haploid chromosomes random orientation results in independent assortment

Telophase I Chromosomes are segregated into two clusters; one at each pole. Nuclear membrane re-forms around each daughter cell. Sister chromatids are no longer identical due to crossing over.

The two newly divided cells DO NOT undergo REPLICATION before MEIOSIS II, they Simply continue to the second meiotic division. NO INTERPHASE II

Second Meiotic Division Meiosis II resembles normal mitotic division. prophase II - nuclear envelope breaks down and second meiotic division begins metaphase II - spindle fibers bind to both sides of centromere and line up in the middle anaphase II - spindle fibers contract and sister chromatids move to opposite poles telophase II - nuclear envelope re-forms and chromosomes uncoil Final result - four haploid cells

PROPHASE II METAPHASE II

ANAPHASE II TELOPHASE II

A Comparison of Mitosis and Meiosis Meiosis and mitosis can be distinguished from mitosis By three events in Meiosis l

1. Synapsis and crossing over Homologous chromosomes physically connect and exchange genetic information. Chromatids are not identical.

2. Tetrads on the metaphase plate At metaphase I of meiosis, paired homologous chromosomes (tetrads) are positioned on the metaphase plates TETRADS

3. Separation of homologues At anaphase I of meiosis, homologous pairs move toward opposite poles of the cell In anaphase II of meiosis, the sister chromatids separate.

(before chromosome replication) MITOSIS MEIOSIS Prophase Duplicated chromosome (two sister chromatids) Chromosome replication Parent cell (before chromosome replication) Chiasma (site of crossing over) MEIOSIS I Prophase I Tetrad formed by synapsis of homologous chromosomes Metaphase Chromosomes positioned at the metaphase plate Tetrads Metaphase I Anaphase I Telophase I Haploid n = 3 MEIOSIS II Daughter cells of meiosis I Homologues separate during anaphase I; sister chromatids remain together Daughter cells of meiosis II n Sister chromatids separate during anaphase II Anaphase Telophase Sister chromatids separate during anaphase 2n Daughter cells of mitosis 2n = 6

SIGNIFICANT RESULTS OF MEIOSIS: Four haploid cells are produced because two rounds of division follow one round of chromosome replication = sex cells or gametes Daughter cells are NOT identical to their parent(s) like in Mitosis Causes genetic variation (recombination) in offspring

Ways in which Meiosis creates Genetic Variation a. crossing over - recombination b. independent assortment of chromosomes into gametes c. Random union of gametes to form a zygote

Independent Assortment

Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!!

Formation of Gametes In humans meiosis occurs in the testes and ovaries. In the testes, meiosis results in the production of four sperm = SPERMATOGENESIS In the ovaries, meiosis results in the production of one egg and three polar bodies that disintegrate. Thus, all the cytoplasm goes to the one egg cell and it is larger than sperm = OOGENESIS

Mutations A mutation is a permanent change in the DNA sequence of a gene. Sometimes mutations in DNA can cause changes in the way a cell behaves. This is because genes contain the instructions necessary for a cell to work. If some of the instructions to the cell are wrong, then the cell may not know what it is supposed to do!

There are two ways in which DNA can become mutated: 1) Mutations can be inherited. This means that if a parent has a mutation in his or her DNA, then the mutation is passed on to his or her children – GERM CELL MUTATION 2) Mutations can be acquired. This happens when environmental agents damage DNA, or when mistakes occur when a cell copies its DNA prior to cell division – SOMATIC CELL MUTATION

Mutations may occur on the whole chromosome or within a single gene. CHROMOSOME MUTATIONS Changes in the structure of a chromosome Types: 1) deletion – loss of a piece of chromosome due to chromosome breakage. 2) inversion – chromosome breaks off and then reattaches to the same chromosome 3) translocation – chromosome breaks off and reattaches to a different chromosome 4) nondisjunction – failure of chromosome to separate from its homologue during meiosis

CHROMOSOME MUTATIONS Changes in the structure of a chromosome Types: 1) deletion – loss of a piece of chromosome due to chromosome breakage. 2) inversion – chromosome breaks off and then reattaches to the same chromosome 3) translocation – chromosome breaks off and reattaches to a different chromosome 4) nondisjunction – failure of chromosome to separate from its homologue during meiosis

Lead to abnormal conditions like Down’s syndrome – extra Chromosome #21

Gene Mutations Involve large segments of DNA or a single nucleotide within a codon. Types 1. Point mutation – substition, addition, or removal of a single nucleotide 2. Frame shift – addition or deletion of a single nucleotide Much more harmful because affects entire protein