11.4 MeiosisVocabulary: Homologous, diploid, haploid,

meiosis, tetrad, crossing-over

Key Concepts:What happens during the process of meiosis?How is meiosis different from mitosis?

Types of ReproductionSection 11-4

ASEXUAL SEXUAL

1. One Parent

2. Binary fission (in prokaryotes) or mitosis in (eukaryotes).

3. Offspring are a clone of the parent. Not genetically different.

1. 2 parents.

2. Results in union of sex cells (egg & sperm or pollen in plants) Sex cells are called gametes.

3. Offspring not clones of parent. Genetically different.

METHODS OF ASEXUAL REPRODUCTION

SEXUAL REPRODUCTION1. Chromosomes come in pairs called HOMOLOGOUS PAIRS.

A. Remember – one of each pair is from each parent.

B. Humans have 23 homologous pair.

2. 1 – 22 pairs are called autosomes. Normal body cells (muscle, nerve, bone, skin etc) have 23 pairs of chromosomes.3. 23rd pair (either XX or XY) are called sex chromosomes

4. Normal body cells are Diploid (2N) and sex cells are Haploid (1N) (half of Diploid)

EXAMPLES IN ORGANISMS OF 1N & 2N

1. A cat has 38 chromosomes (2N) in its normal body cells. Its sex cells (sperm or egg) has half this # (1N) (19). A chicken has 39 chromosomes in its sex cells (1N) thus it has 78 in its normal body cells.

2. Thus when in humans sperm (23 chromosomes) meets the egg (23 chromosomes) the chromosome number of the fertilized egg (called a zygote) is 46. Thus 1N & 1N = 2N.The fertilized egg then divides by mitosis (asexual reproduction) to keep all somatic (body) cells at 46.

3. Thus, their most be another type of cell division that cuts the chromosome number in half . This division is called MEIOSIS.

Example of how gametes join together to make a fertilized egg (zygote.)

Section 11-4

Figure 11-15 Meiosis

Meiosis I

Meiosis II

Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original.Prophase II Metaphase

IIAnaphase II Telophase II

The chromosomes line up in a similar way to the metaphase stage of mitosis.

The sister chromatids separate and move toward opposite ends of the cell.

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

Section 11-4

Note how crossing over happens and how the final 4 cells only have one of each homologous pair.

Parent or mother cell

4 Daughter cells

Note that sperm production usually produces 4 viable (usable) sperm while females only produce 1 usable egg.

MOST IMPORTANT PURPOSES OF MEIOSIS

1. Each sex cell (gamete) receives 1 CHROMOSOME FROM EACH HOMOLOGOUS (MATCHING) PAIR NEVER BOTH!!

2. Meiosis creates genetic variation in organisms by:

A. Chromosomes assort themselves in gametes independently.

B. Different chromosome combinations possible in a sex cell

C. Crossing over can produce even more genetic variation.

Section 11-4

Crossing-OverHOW CROSSING OVER CAN HAPPEN – DURING PROPHASE I

REMEMBER – This does not have to happen but it can!