Chapter 23 Meiosis 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

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 Meiosis First DivisionSecond 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.

Second Meiotic 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

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

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).

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.