Chapter 10 Sexual Reproduction

& GeneticsPart 1: Meiosis

Concepts for Concepts for ReviewReview• Chromatin is the relaxed form of

DNA• Chromosome is the coiled form of

DNA• Sister chromatids are identical copies

of DNA (after S-phase/replication)

Key Terms Key Terms • Homologous chromosomes are

chromosomes that make up a pair, one from each parent– Not necessarily identical but…– Are the same length– Have the same genes

Key Terms Key Terms • Diploid refers to complete

chromosome sets in a cell (or 2N)– N= the number of chromosomes in a

gamete (sex cell)– N chromosomes from the female

parent + N chromosomes from the male parent= 2N

– For humans, 2N = 46

Key Terms Key Terms • Haploid refers to single

chromosomes in a cell (or N)– One-half of the 2N set (the maternal

N or the paternal N)– Gametes (i.e. sperm, egg sex cells) are

haploid cells– For humans, N = 23

Bottom Line

• diploid means that there are the full number of chromosomes where as haploid means that there are half the number of chromosomes.

Key Terms Key Terms • Meiosis is a type of cell division

that reduces the number of chromosomes – Forms gametes (i.e. sex cells)– Occurs in the reproductive structures

of organisms that reproduce sexually– Occurs in 2 consecutive cell divisions:

meiosis I and meiosis II

Before meiosis Before meiosis Interphase occurs….Interphase occurs….• Replication of DNA occurs.• Chromosomes Condense

Sounds familiar right???

Meiosis I: Meiosis I: “Reduction “Reduction Division”Division” Reducing

2N cells

to…

N cells

PROPHASE I• Homologous

chromosomes pair up• Crossing over occurs• Nuclear envelope

breaks down• Spindles form

Crossing Over

• Chromosomal segments (genetic information) are exchanged between a pair of homologous chromosomes

METAPHASE I

• Spindle fibers attach to the centromeres

• Homologous Chromosomes Line up at the equator

ANAPHASE I

• Homologous chromosomes separate and move to the opposite poles of the cell

TELOPHASE I

• The cell divides• The result is two

daughter cells. • Each daughter cell is

haploid.• The daughter cells are

not genetically identical

**During Telophase I Cytokinesis occurs at the same time!!**

Meiosis II: “Mitotic Meiosis II: “Mitotic Division”Division”

Separatin

g sister

chromatid

s

• Nucleus breaks up• Spindles form and

attach to centromere

• Sister chromatids randomly align at middle of cell

• Sister chromatids separate

• Chromosomes move to opposite poles of cell

• Cells divide• 4 haploid

daughter cells result

• Each cell is genetically different

Summary of Key Summary of Key ConceptsConcepts• Chromosomes contain genes (humans

have 46 chromosomes or 23 homologous pair)

• Homologous chromosomes may contain different forms of the same genes

• Meiosis reduces chromosome # by ½ and results in sex cells (gametes)

• Crossing-over increasing genetic variation

Click here for detailed animation of meiosis

which set of chromatids illustrates the result of a single crossover of the homologous chromosomes?