Sexual Reproduction &Mendelian Genetics

Chapter 10Pages 270-285

First, Some Vocabulary……somatic cells – body cells, 46 chromosomes in

humans

gametes – sex cells, have half the number of chromosomes as compared to somatic cells, so 23 chromosomes in human gametes

diploid = 2n number of chromosomes

haploid = n number of chromosomes

Fertilization – the process by which one haploid gamete combines with another haploid gamete.

oogenesis – the creation of an egg

spermatogenesis – the creation of sperm

Meiosis

Section 10.1Pages 270-276

Meiosis:

• Is the type of cell division that makes gametes.• Occurs in reproductive cells in which one diploid

cell (2n) produces four haploid cells (n) that are not genetically identical.

• Is referred to as reduction division because it reduces the number of chromosomes from 2n to n.

• Involves two consecutive cell divisions called Meiosis I and Meiosis II.

Meiosis

Mitosis vs. Meiosis

• Meiosis occurs in the cells of the reproductive organs to produce gametes (egg and sperm cells).

• Mitosis occurs in body cells.• Each body cell has (2n) chromosomes after

mitosis.• Each egg and sperm cell has (n) chromosomes

after meiosis

Haploid vs. Diploid

• Egg and sperm cells are haploid (n) because they have HALF the # of chromosomes found in body cells.

• Body cells are diploid (2n) because they have 2 sets of chromosomes.

• When egg and sperm cells fuse = zygote has (2n) chromosomes.

Meiosis Animation

• http://highered.mcgraw-hill.com/olc/dl/120074/bio19.swf

Meiosis I

Interphase I Prophase I Metaphase I Anaphase I

DNA replicates. Chromatin condenses.

Synapsis – pairing of homologous chromosomes to form a tetrad (4 chromatids).

- Crossing over occurs

Homologous chromosomes line up along the equator of the cell.

Homologues separate and get pulled to opposite poles by spindle fibers.

Telophase I (not shown) – the cell divides into two haploid cells (see pg 273 of text).

Crossing Over in Prophase I leads to genetic variation:

Notice the grey and white chromosomes have exchanged segments. This change occurs during synapsis and is called crossing over. Crossing over is a process during which chromosomal segments are exchanged between a pair of homologous chromosomes.

Meiosis II

Meiosis I results in two haploid (n) daughter cells, each with half the number of chromosomes as the parent cell.

Prophase II Metaphase II Anaphase II Telophase IIThe chromosomes line up on the metaphase plate (or equatorial plate).

Sister chromatids separate and move toward opposite ends of the cell.

Meiosis II results in four haploid (n) daughter cells.

Comparing Mitosis to Meiosis

• Mitosis= results in 2 genetically identical, diploid (2N) cells.

• Meiosis= results in 4 genetically different, haploid (N) cells.

Comparing Meiosis to MitosisMEIOSIS I

• Prophase I– Homologous pairs form tetrads

by synapsis.– Crossing over occurs.– Nuclear envelope breaks down, spindles

form.• Metaphase I

– Tetrads line up on the equator.

• Anaphase I– Homologues separate.

• Telophase I– New nuclei form and contain the

n amount of DNA.• Meiosis II is just like mitosis.

MITOSIS• Prophase

– Chromosomes condense– Nuclear envelope breaks down – Spindles form

• Metaphase– Chromosomes line up on equator

• Anaphase– Sister chromatids separate

• Telophase– New nuclei form with normal

chromosomes

Review Questions For YOU!!

1. Why is it important for chromatin to condense?

2. Describe chromatin vs. chromosome.

3. Define the term “autosome”.

Meiosis – Spermatogenesis:

• Male - sperm cells = Meiosis results in 4 viable sperm cells.

chromosomes

chromosomes

chromosomes

Meiosis - Oogenesis

• Female - egg cells = one egg cell gets most of the cytoplasm and the other 3, called polar bodies, disintegrate.

Chromosomes

Chromosomes

Chromosomes

Nondisjunction:

A. Normal Meiosis

B. Failure of homologous chromosome pair to separate during Meiosis I

C. Failure of sister chromatids to separate during Meiosis II

Trisomy 21 – Down’s Syndrome

Sexual Reproduction vs. Asexual Reproduction