Cells For Sexual Reproduction Meiosis Cells For Sexual Reproduction

Ch. 11.4 Meiosis for Sexual Reproduction Sexual Reproduction - two parents a. Offspring have NEW combination of genes b. genetic mix of both parents Advantage – genetic variation a. Some may have traits that favor survival b. Variation in individuals allows a species to evolve 

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

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 pairs autosomes; one pair sex chromosomes X, Y Gene for one trait

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

Chromosomes photographed during mitosis Humans: 2n = 46 23 pairs of chromosomes One set of 23 came from each parent Chromosomes photographed during mitosis

Haploid cells have ONE set of chromosomes (n) = ONE MEMBER from EACH homologous PAIR human: n = 23 gametes - sperm or egg

Meiosis is “Reduction Division” Reduces chromosome number from diploid to haploid Cell DIVIDES TWICE

Meiosis - two cell divisions 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) Meiosis I - Pairs separate (n = 1) Meiosis II - copies separate (n = 1) Haploid cells - (n = 1)

Crossing over – only in meiosis a. during Prophase or Metaphase I b. Homologous chromatids trade pieces c. Increases genetic variation

Two equally probable arrangements of chromosomes at metaphase I INDEPENDENT ASSORTMENT – Each chromosome pair lines up randomly in meiosis, can separate differently Combination 1 Combination 2 Combination 3 Combination 4 Gametes Metaphase II Two equally probable arrangements of chromosomes at metaphase I Possibility 1 Possibility 2 Figure 8.16

Meiosis I Prophase I Metaphase I Interphase Anaphase I Telophase I Cytokinesis

Meiosis II Haploid (n) cells Doubled chromosomes Prophase II Metaphase II Telophase II Anaphase II Haploid cells Single chromosomes

Compare mitosis and meiosis

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

Ovum needs all the cytoplasm Sperm needs only nucleus(DNA) - flagellum - mitochondria for energy Ovum and polar body

Causes 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 n pairs  2n 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

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

Accidents during meiosis  wrong chromosome number Nondisjunction in meiosis I Normal meiosis II Gametes n + 1 n 1 Number of chromosomes Nondisjunction in meiosis II Normal meiosis I n -1 n Number of chromosomes Nondisjunction in meiosis I Nondisjunction in meiosis II All gametes abnormal Some gametes normal

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

KARYOTYPE picture of a person’s chromosomes Photographed during mitosis - sorted into homologous pairs - largest-to-smallest - sex chromosomes last

Normal female karyotype Normal male karyotype Normal female karyotype

Abnormal KARYOTYPE Look for: - missing or extra pieces - pieces broken or moved - pieces added or lost autosomes sex chrom. Trisomy 21

Down Syndrome Trisomy chromosome # 21

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

Klinefelter Syndrome XXY

Other chromosomes mutations