MEIOSIS Ch. 8 CELLS FOR SEXUAL REPRODUCTION

Meiosis for Sexual Reproduction Sexual Reproduction - two parents a. Offspring are genetic mix of both parents b. Have a NEW combination of genes Advantage – genetic variation in offspring a. Some may have traits that favor survival b. Can pass these traits on to offspring c. Darwin’s theory - “ survival of the fittest” d. Variation in individuals allows a species to evolve

CONJUGATION a. Recipient cell gets new genes b. Bacteria and protists Sexual Reproduction in bacteria and protists

Complex organisms – make special cells a. gametes – sperm and egg b. Gametes combine in fertilization - make a zygote  new organism

Chromosome Number: Diploid and Haploid Homologous chromosomes a. matched chromosome pairs b. one member of pair from each parent c. carry genes for the same traits d. 22 autosome pairs; one pair sex chromosomes X, Y Gene for one trait

Cells with paired chromosomes are diploid a. 2n (n = number) b. Human: 2n = 46 (23 pairs) b. Somatic (body) cells are diploid Fruit fly 2n = 8 2 sets of chromosomes - 2 of every gene Locus – location of gene on a chromosome

Haploid Cells (n) Gametes – sperm and egg only one set of chromosomes, one from each pair somatic cell sex cell

Meiosis is “Reduction Division” REDUCES chromosome number by half Haploid (n) - one set of chromosomes -human: 2n = 46 n = 23

Meiosis Reduces the Chromosome Number 2n parent cell DNA replicates in interphase First division – pairs separate Second division – sister chromatids separate  4 haploid daughter cells

Homologous pairs separate in MEIOSIS TWO cell divisions - Daughter cells have ½ parent chromosome number Diploid cell - Has pairs (2n = 2) Haploid cells - (n = 1) Meiosis I - Pairs separate (n = 1) Meiosis II - copies separate (n = 1)

Crossing over – only in meiosis a. In Meiosis I b. Homologous chromatids trade pieces c. Increases genetic variation

Meiosis I: homologous pairs separate - makes two daughter cells, but sister chromatids are still attached MEIOSIS I: Homologous chromosomes separate INTERPHASE PROPHASE I METAPHASE I ANAPHASE I Centrosomes (with centriole pairs) Sites of crossing over Spindle Microtubules attached to kinetochore Metaphase plate Sister chromatids remain attached Nuclear envelope Chromatin Sister chromatids Tetrad Centromere (with kinetochore) Homologous chromosomes separate 2n parent cell synapsis pairs line up pairs separate disjunction

Meiosis II: sister chromatids separate  4 haploid cells PROPHASE II METAPHASE II ANAPHASE II TELOPHASE I AND CYTOKINESIS TELOPHASE II AND CYTOKINESIS Cleavage furrow Haploid daughter cells forming Sister chromatids separate MEIOSIS II: Sister chromatids separate 2n  n two daughter cells chromatids 4 daughters one chromosome set each separate one set two copies (sisters) disjunction single copies

8.15 Review: Comparing mitosis and meiosis MitosisMeiosis Parent cell (before chromosome replication) Chromosome replication Chromosomes align at the metaphase plate Tetrads align at the metaphase plate Sister chromatids separate during anaphase Homologous chromosomes separate during anaphase I; sister chromatids remain together No further chromosomal replication; sister chromatids separate during anaphase II Prophase Metaphase Anaphase Telophase Duplicated chromosome (two sister chromatids) Daughter cells of mitosis 2n2n 2n2n Daughter cells of meiosis I n n n n 2n = 4 Tetrad formed by synapsis of homologous chromosomes Meiosis i Meiosis ii Prophase I Metaphase I Anaphase I Telophase I Haploid n = 2 Daughter cells of meiosis II 2n copies 2n single 2n copies n copies n single

Sources of genetic variation 1. Homologous pairs have different genes same traits, but may be different forms 2. Crossing over – homologs trade pieces before separating  new gene combinations 3. Pairs position in Metaphase I – independent of each other n pairs  2 n possible combinations 4. Random fertilization of eggs by sperm Any egg or sperm is equally likely to be used 5. Gene or chromosome mutation - Error in replication or cell division

–“Independent Assortment” –Many different gene combinations in haploid gametes Combination 1 Combination 2 Combination 3 Combination 4 Gametes Metaphase II Two equally probable arrangements of chromosomes at metaphase I Possibility 1Possibility 2 Figure Chromosomes line up randomly in meiosis

Making sperm and egg Sperm: 2n parent cell  4 haploid sperm Ovum: 2n parent cell  1 haploid egg + haploid polar bodies

Ovum needs all the cytoplasm Ovum and polar body (0.1mm) Sperm needs only DNA - and flagellum - and mitochondria for power - and acrosome to penetrate ovum

Fertilization restores diploid

What happens after fertilization?

Cleavage: mitotic divisions in early embryo reduce cell size, until normal Cells differentiate  new organism

When meiosis goes wrong Nondisjunction - do not separate correctly In mitosis  defective nucleus, cell usually dies In meiosis  defective gamete  wrong number of chromosomes in zygote

8.21 Accidents during meiosis can alter chromosome number Nondisjunction in meiosis I Normal meiosis II Gametes n  1 n  1 Number of chromosomes Nondisjunctio n in meiosis II Normal meiosis I Gametes n  1n  1 n n Number of chromosomes Nondisjunction in meiosis I Nondisjunction in meiosis II All gametes abnormal Some gametes normal

Fertilization after nondisjunction  trisomy in zygote Sperm cell Egg cell n (normal) n + 1 Zygote 2n + 1 Abnormal chromosome number = aneuploidy Trisomy = 3 Wrong chromosome number in zygote  wrong number in every cell in organism

KARYOTYPE picture of a person’s chromosomes Photographed during mitosis - sorted into homologous pairs - largest-to-smallest - sex chromosomes last Abnormalities visible: - missing or extra - pieces broken or moved - pieces added or lost autosomessex chrom. Trisomy 21

Normal male karyotype Normal female karyotype

Down Syndrome Trisomy chromosome # 21

8.22 Abnormal number of sex chromosomes usually do not affect survival in humans Nondisjunction of large chromosomes is usually lethal Down Syndrome - # 21 is very small, carries few genes In sex chromosomes, leads to varying degrees of malfunction, but usually not lethal

Turner Syndrome XO Figure 8.22B Characteristic facial features Web of skin Constriction of aorta Poor breast development Under developed ovaries

Turner Syndrome

Klinefelter Syndrome XXY

Klinefelter Syndrome

Abnormalities of Sex Chromosomes in Humans

Other chromosome changes can cause birth defects or cancer Chromosomes break – pieces lost or rearranged - in somatic cells  increases cancer risk - in gametes  genetic disorders