Chapter 10 2 Meiosis and Genetic Variation 1

Chapter 10. 2 - Meiosis and Genetic Variation 1

The Human Genome • Genome: Complete complement of an organism’s DNA. – Includes genes (control traits) and non -coding DNA organized into chromosomes. 2

Genes • Eukaryotic DNA is organized in chromosomes. – Genes have specific places on chromosomes. 3

Heredity • Heredity – way of transferring genetic information to offspring • Chromosome theory of heredity: chromosomes carry genes. • Gene – “unit of heredity”. 4

All sexually reproducing organisms have two types of cells in their bodies • Diploid cells • – Normal body cells – Produced during mitosis – 2 sets of chromosomes paired together (2 n) Haploid Cells – Sex cells (gametes) – Produced during meiosis – Only 1 set of chromosomes (n) 5

Diploid cells A normal diploid human body cell has 46 chromosomes paired together (23 pairs) • The paired chromosomes match each other in gene type and location • They are called homologous pairs 6

Problem-solving Lab 10. 2 on page 264. • • • Answer in your notes 1) 2) 3) 4) 7

• Looking at Chromosomes in a cell Karyotype: – ordered display of an individual’s chromosomes – Chromosomes are stained to reveal visible band patterns and major abnormalities. 8

Karyotyping • Shows the homologous pairs of chromosomes • Identify sex of offspring • Identify extra or missing chromosomal disorders 9

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Chromosomal disorders • Trisomy - Extra chromosome so 3 instead of 2 • Monosomy - Missing a chromosome so 1 instead of 2 Down’s Syndrome (trisomy 21) 11

Genes are passed on through Reproduction • Asexual – Without sex cells – Produces identical copies of parent (clones) – Mitosis is asexual reproduction • Sexual – With sex cells – Produces genetically variable offspring • This is good for survival! – Meiosis is sexual reproduction 12

Asexual Reproduction • single-celled organisms reproduce by splitting, budding, parthenogenesis. • offspring are genetically identical to parent. 13

Sexual reproduction • Important vocabulary – Gametes = sex cells (egg and sperm) that are haploid – Haploid = one set of chromosomes – Diploid = two sets of chromosomes – Zygote = fertilized egg • Fusion of two gametes to produce a single fertilized egg (zygote). 14

Meiosis (sexual) vs. Mitosis (asexual) • Meiosis reduces the number of chromosomes by half. – Produces 4 haploid cells • Daughter cells differ from parent cell • Meiosis involves two divisions • Mitosis keeps the same number of chromosomes – Produces 2 diploid cells • Daughter cells identical to parent (clones) • Mitosis involves only one division 15

Sexual Reproduction is Important! • Because a zygote has genes from two different parents…. . – Introduces greater genetic variation for a species – Allows for genetic recombination – Increase species survival! 16

Meiosis I - First division of meiosis • Interphase - all chromosomes replicate (just like in mitosis) • Prophase 1: Homologous chromosomes begin to pair up. Crossing-over can occur during the latter part of this stage. 17

Crossing over - occurs during Prophase I of meiosis Genetic recombination that is an exchange of genetic material between homologous chromosomes Crossing over produces recombinant chromosomes and increases genetic variation! 18

Harlequin chromosomes crossing over 19

Meiosis I continued • Metaphase 1: Homologous chromosomes align at the equatorial plate. (in pairs next to each other) • Anaphase 1: Homologous pairs separate with sister chromatids remaining together. • Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair. 20

Meiosis II Second division of meiosis: Gamete formation • Prophase 2: DNA does not replicate again. • Metaphase 2: Chromosomes align at the equatorial plate. • Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. • Telophase 2: Cell division is complete. Four haploid daughter cells are obtained. 21

Meiosis creates genetic variation • Meiosis results in genetic variation by shuffling of maternal and paternal chromosomes and crossing over. *No daughter cells formed during meiosis are genetically identical to either mother or father *During sexual reproduction, fusion of the unique haploid gametes produces truly unique offspring. 22

Independent assortment - chromosomes separate randomly during meiosis 23

In humans e. g. 23 chromosomes in haploid 2 n = 46; n = 23 2 n = 223 = ~ 8 million possible combinations! 24

Random fertilization At least 8 million combinations from Mom, and another 8 million from Dad … >64 trillion combinations for a diploid zygote!!! 25

Meiosis & sexual life cycles • Life cycle = sequence of stages in organisms reproductive history; conception to reproduction. • Somatic cells = any cell other than gametes, most of the cells in the body. • Gametes produced by Generalized animal life cycle meiosis. 26

Sex is costly! • Large amounts of energy required to find a mate and do the mating: specialized structures and behavior required • Intimate contact provides route for infection by parasites (AIDS, syphillis, etc. ) • Genetic costs: in sex, we pass on only half of genes to offspring. • Males are an expensive luxury - in most species they contribute little to rearing offspring. 27

But … • More genetic diversity: more potential for survival of species when environmental conditions change. – Shuffling of genes in meiosis – Crossing-over in meiosis – Fertilization: combines genes from 2 separate individuals • DNA back-up and repair. – Asexual organisms don't have back-up copies of genes, sexual organisms have 2 sets of chromosomes and one can act as a back-up if the other is damaged. – Sexual mechanisms, especially recombination, are used to repair damaged DNA - the undamaged chromosome acts as a template and eventually both chromosomes end up with the correct gene. 28

Study Questions • 1. What happens as homologous chromosomes pair up during prophase I of meiosis? • 2. How does metaphase of mitosis differ from metaphase I of meiosis? • 3. What is the sole purpose of meiosis? • 4. What specific activities, involving DNA, occur during interphase prior to both mitosis and meiosis? 29

• • • 5. Compare mitosis and meiosis on the following points: a. number of daughter cells produced. b. the amount of DNA in the daughter cells in contrast to the original cell. c. mechanism for introducing genetic variation. 6. What is a zygote and how is it formed? 7. What is the main advantage of sexual reproduction? 30

Meiosis – key differences from mitosis • Meiosis reduces the number of chromosomes by half (haploid cells). • Daughter cells differ from parent cell and each other. • Meiosis involves two divisions, Mitosis only one. • Meiosis produces 4 haploid cells, Mitosis produces 2 diploid cells. 31