Reproduction Ch.7

Asexual Reproduction One parent The prokaryotic chromosome is a ring of DNA Binary fission-bacteria – Cell splits in two – Produces two genetically identical daughter cells, exactly like the original – Identical offspring (clones) Different kinds – Binary fission - bacteria – Budding – yeast, sponges – Fragmentation – sea stars

Binary Fission Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 1. Attachment of chromosome to a special plasma membrane site indicates that this bacterium is about to divide. 2. The cell is preparing for binary fission by enlarging its cell wall, plasma membrane, and overall volume. 3. DNA replication has produced two identical chromosomes. Cell wall and plasma membrane begin to grow inward. 4. As the cell elongates, the chromosomes are pulled apart. Cytoplasm is being distributed evenly. 5. New cell wall and plasma membrane has divided the daughter cells. chromosome cell wall plasma membrane cytoplasm SEM2,345X © Dennis Kunkel Microscopy, Inc./Visuals Unlimited

Sexual Reproduction Two parents Sexual Reproduction – Cell splits twice – Produces four daughter cells, genetically different from the parent and each other – meiosis Increases genetic variation

Haploid vs. Diploid HAPLOID 1n One copy of each chromosome DIPLOID 2n Two copies of each chromosome Homologous chromosomes-similar in size, shape and genetic content

Homologous Chromosomes 6 a. Sister chromatids duplication chromosome maternal chromosome b. paternal chromosome homologous pair nonsister chromatids centromere kinetochore chromosome Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. a: © L. Willatt/Photo Researchers, Inc. Homologous chromosomes-similar in size, shape and genetic content

How does Meiosis work? Two stages, Meiosis I and Meiosis II Meiosis I : Homologous chromosomes are separated from each other – Chromosomes replicate first, so each chromosome has a “twin” – Example: this is where the X and Y chromosomes separate from each other and go into different cells X chromosome is a double X, Y chromosome is a double Y – Two new cells have 23 duplicated chromosomes

Crossing over-during Prophase I – Homologous chromosomes exchange genes while they’re lined up next to each other – You don’t inherit entire chromosomes from your parents—only pieces of them – Creates more variation and diversity within a species – Important for natural selection and evolution

2. Crossing Over Homologous chromosomes swap genes Happens during Prophase I Increases genetic variation even more

Meiosis II – Just like mitosis, but with 23 chromosomes instead of 46 Remember these are 23 duplicated chromosomes – Starts with 2 cells, which divide into 4 – In males: all 4 cells become sperm – In females: one cell becomes an egg, the other 3 die

What’s a gamete? Female gamete = egg (ovum); produced in ovaries; oogenesis Male gamete = sperm; produced in testes; spermatogenesis

Overview of Meiosis 14 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. 2n = 4 Diploid parent cell

Overview of Meiosis 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. centrioles nucleolus centromere MEIOSISI Homologous pairs synapse and then separate. 2n = 4 chromosome duplication

Overview of Meiosis 16 centrioles nucleolus centromere sister chromatids MEIOSISI Homologous pairs synapse and then separate. MEIOSISII Sister chromatids separate, becoming daughter chromosomes. n = 2 Four haploid daughter cells Second divisionFirst division synapsis 2n = 4 chromosome duplication n = 2 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Comparison of Mitosis & Meiosis Mitosis Results in 2 daughter cells Daughter cells are diploid Daughter cells are identical to each other and to parent Meiosis Results in 4 daughter cells Daughter cells are haploid Daughter cells are different from each other and from parent

Mitosis v. Meiosis

Mechanisms for Increasing Genetic Variation

1. Independent Assortment Each of the 23 pair of chromosomes separates into gametes independently (2 23 different gametes can form=8 million possible gamete combinations)

2. Crossing Over Homologous chromosomes swap genes Happens during Prophase I Increases genetic variation even more

3. Random Fertilization For humans - 23 pairs of chromosomes: Fertilization of an egg by a sperm is random # possible outcomes 2 23 (egg) X 2 23 (sperm)= 64 trillion

Determination of Sex Females: two X chromosomes (XX) Males: XY All offspring will receive an X chromosome from the mother and either an X or a Y from the father The X chromosome is large and contains over 1,000 genes The Y chromosome is tiny and contains less than 200 genes