Chapter 23Meiosis and

Sexual Reproduction

Asexual Reproduction (review)

• Single parent gives rise to new offspring by mitotic cell division

• Each new individual receives a set of chromosomes identical to the parent chromosomes

• No variation of hereditary information

(I) Sexual Reproduction• Two parents give rise to new offspring by

the fusion of nuclear materials from two different cells

• Offspring are not identical to the parent

• Variations exist, which increases the species ability to adapt to the changing environment

(A) Gonads

• Sex organs

1. Males – Testes

2. Females – Ovaries

(A) Gametes

• Sex cells

1. Males – sperm

2. Females – ovum

(B) Fertilization

• The fusion of the nuclei of one sperm with one ova (egg) to

produce a zygote

• Sperm (n) + Ovum (n) =2n

• Sperm (23) + Ovum (23) = 46

(C) Haploid• Also known as monoploid• Represented by: n• The gametes contain half the number of

chromosomes• Remember, gametes are sex cells that

combine to form new offspring. Therefore gametes are haploid and once they fuse (combine), they form a zygote that is diploid

(D) Diploid

• Contain the full number (set) of chromosomes

• Represented by: 2n

Homologous Chromosomes

• Pairs of similar chromosomes

• Humans have 46 chromosomes:

22 pairs of homologous chromosomes

1 pair of sex chromosomes

(II) Meiosis• Known as Reduction Division• Meiosis is a type of cell

division in which the daughter cell receives only half the number of chromosomes present in the parent cell

SACCONE IS THE COOLEST

23 Pairs of chromosomes of a human cell

• The chromosomes labeled X and Y are the sex chromosomes

• XX = female

• XY = male

(III) Stages of Meiosis• In meiosis, each cell divides twice:

1. The first stage is very similar to mitosis

2. The second stage is also like mitosis with the exception that there is

NO replication of chromosomes in the second phase (meiosis 2)

Phases of MeiosisFirst Division Second Division

Prophase I

Metaphase I

Anaphase I

Telophase I

Prophase II

Metaphase II

Anaphase II

Telophase II

Key

SS = single-stranded

DS = double-stranded

First Meiotic Division

Prophase I • Replication of SS chromosomes

into DS chromosomes has already occurred.

• Each chromosome then pairs up with it’s homologous chromosome.

• This is known as synapsis.

Prophase I • Each chromosome pair consists

of 4 chromatids and thus is a tetrad.

• Crossing over occurrs when segments of DNA are exchanged between the chromatids found in tetrads.

Metaphase I

• Alignment of the tetrads (4 chromatids) in the middle of the

cell (equatorial plane).

Anaphase I

• The homologous chromosomes separate here.

• Chromrsomes are still double stranded.

• The process of separating the homologs is known as disjunction.

Telophase I

• Cytokinesis occurs as in Mitosis however each of the daughter cells

has half the # of chromosome compared to the parent.

• After this point, the chromosomes WILL NOT replicate.

• Cells usually start prophase 2 immediately after telophase 1.

SecondMeiotic Division

Prophase II

• Chromosomes DO NOT replicate.

• DS chromosomes move towards the equatorial plane.

Metaphase II

• Tetrads line up in the MIDDLE of the cell.

• This is sometimes called the equatorial plane or the

metaphase plate.

Anaphase II• DS chromosomes

separate. • SS chromosomes move

to opposite ends.

Telophase II

• Both daughter cells undergo cytokinesis forming 4 haploid

cells.

• Thus, each cell has ½ the number of chromosomes as the parent.

Meiosis

• Diploid (2n) cells divide

haploid (n) cells

These cells mature into specialized reproductive cells (sperm and ova).

(III) Sexual Reproduction

in

Animals

(A) Reproductive System• Gonads- specialized organs that

produce gametes a) ovaries - female gonad - produce ova (egg) b) testes - male gonad - produce sperm cell

(B) Hermaphrodite

• Contain both male and female reproductive structures

(C) Gametogenesis• Process by which gametes are

produced in the gonads

• Two types:

Spermatogenesis

Oogenesis

SACCONE IS THE COOLEST

Spermatogenesis• Primary spermatocyte

goes through first meiotic division and form two daughter cells of equal size

• Each daughter cell (secondary spermatocyte) goes through the second meiotic division forming four motile sperm cells of equal size

SACCONE IS THE COOLEST

Oogenesis• The primary oocyte goes

through first meiotic division and the cytoplasm of the cell is divided unequally

• The larger daughter cell is called the secondary oocyte and the smaller daughter cell is called the polar body

• During second meiotic division, 1 mature ovum is produced and 3 polar bodies

Comparison of Ova and Sperm

• Egg cells contain stored food in the form of yolk.

• Egg cells are larger than sperm.

• Sperm are motile, egg cells sessile (can’t move on their own).

SACCONE IS THE COOLEST

Fertilization• Union of a haploid

sperm nucleus with a haploid egg nucleus

• This results in a diploid zygote with the full number of chromosomes

Two types of Fertilization

Internal Fertilization

External Fertilization

Internal Fertilization

• Takes place inside the body of the female

• Less eggs are required

Ex: mammals and birds

External Fertilization• Eggs are fertilized outside the body of

the female

• Large number of eggs are required

• This type of fertilization is found in an aquatic environment

Ex: fish, frogs, & salamanders.