C HROMOSOMES AND M EIOSIS

T WO MAJOR TYPES OF CELLS You have many types of specialized cells in your body, but they can be divided into two major groups: somatic cells germ cells

S OMATIC C ELLS Somatic cells, also called body cells, make up most of your body tissues and organs. Your spleen, kidneys, and eyeballs are all made entirely of body cells. DNA in your body cells is NOT passed on to your children.

G ERM C ELLS Germ cells are cells in your reproductive organs, the ovaries and testes, that develop into gametes. Gametes are sex cells- ova, or eggs, in the female, and spermatozoa, or sperm cells, in the male. Your DNA in your gametes can be passed to your children.

C HROMOSOME N UMBER Each species has a characteristic number of chromosomes per cell. This number is not related to the complexity of the animal.

DNA AND C HROMOSOMES IN B ODY C ELLS Each of your body cells contains a set of 46 chromosomes, which come in two pairs of 23.

C HROMOSOMES Together, each pair of chromosomes is referred to as a homologous pair. “Having the same structure.” Homologous chromosomes are two chromosomes-one inherited from the mother, and one from the father- that have the same length and general appearance. These chromosomes have copies of the same genes, although the two copies may differ.

H UMAN C HROMOSOMES Homologous chromosome pairs are assigned a number to each pair of homologous chromosomes, ordered from largest to smallest. The largest is 1 and the smallest is 23.

H UMAN C HROMOSOMES Collectively, chromosome pairs 1 through 22 make up your autosomes, chromosomes that contain genes for characteristics not directly related to the sex of an organism. Chromosome pair 23 is responsible for determining gender.

C HROMOSOMES Most sexually reproducing species also have sex chromosomes that directly control the development of sexual characteristics. Humans have two very different sex chromosomes. X Y How sex is determined varies by species.

S EX C HROMOSOMES In all mammals, an organism’s sex is determine by the XY system: XX=female XY=male Although the X and Y chromosomes pair with each other, they are not homologous.

S EX C HROMOSOMES X chromosome: The larger sex chromosome and contains numerous genes, including many that are unrelated to sexual characteristics. Y chromosome: A sex chromosome that contains genes that direct the development of the testes and other male traits. It is the smallest chromosomes and carries the fewest genes.

S EXUAL R EPRODUCTION Sexual reproduction is the fusion of two gametes that result in offspring that are a genetic mixture of both parents. The actual fusion of an egg and a sperm cell is called fertilization. When this occurs, the nuclei of the sperm and egg cell fuse to form one nucleus. The new nucleus must have the correct number of chromosomes for a healthy new organism to develop.

D IPLOID Body cells and gametes have different numbers of chromosomes. Your body cells are diploid. What does this mean? This means that a cell has two copies of each chromosome: one copy from the mother, and one copy from the father. They can be represented as 2n. In humans, the diploid chromosome number is 46.

H APLOID Gametes are not diploid cells; they are haploid, represented as n. What does this mean? Haploid means that a cell has only one copy of each chromosomes. Each human egg or sperm cell has 22 autosomes and 1 sex chromosome. In the egg cell, the sex chromosome will always be X. In the sperm cell, the sex chromosome can be either X or Y.

Haploid (n) Diploid (2n)

M EIOSIS Germ cells in your reproductive organs undergo the process of meiosis to form gametes. Meiosis is a form of nuclear division that divides a diploid cell into haploid cells. This process is essential for sexual reproduction.

M EIOSIS

DIVISION OF THE SEX CELLS WHY I DON’T LOOK EXACTLY LIKE BOTH OF MY PARENTS

IF A NEW HUMAN IS CREATED FROM A MOMMY CELL AND A DADDY CELL COMING TOGETHER (EGG & SPERM)….. HOW DO YOU AVOID DOUBLING YOUR CHROMOSOME NUMBERS? PONDER THIS…..

SPERM & EGG UNITE + = 46 chromosomes 92 chromosomes

M EIOSIS REDUCES CHROMOSOME NUMBER AND CREATES GENETIC DIVERSITY. 4 haploid cells made from 1 diploid cell.

One chromosome from each parent, same length, containing similar alleles (genes). Sister chromatids: ½ a chromosome. H OMOLOGOUS C HROMOSOME : C HROMOSOMES THAT ARE SIMILAR (NOT IDENTICAL ) Homologous chromosomes Sister chromatids

C ELLS GO THROUGH 2 ROUNDS OF DIVISION IN M EIOSIS : Meiosis I - divides Homologous chromosomes: Prophase I Metaphase I Anaphase I Telophase I Meiosis II - divides sister chromatids: Prophase II Metaphase II Anaphase II Telophase II Homologous chromosomes Sister chromatids Homologous chromosomes are divided in meiosis1. Sister chromatids are divided in meiosis 2.

M EIOSIS I

1. P ROPHASE I Centrioles move, spindle fibers start to form Chromosomes condense Homologous chromosomes begin to pair up and form TETRAD From Father From Mother

2. M ETAPHASE I Chromosomes align along the center of the cell (note difference in alignment) Each side of cell has chromosomes from both parents Crossing-over occurs

3. A NAPHASE I Paired homologous chromosomes separate from one another Move towards opposite ends Chromatids remain attached

4. T ELOPHASE I Spindle fibers fall apart Cell goes through cytokinesis

MEIOSIS II

5. P ROPHASE II Centrioles and centrosomes move to opposite sides Spindle fibers start to assemble

6. METAPHASE II Chromosomes line up along the middle of the cell. (Note: looks like mitosis now but with ½ of the chromosomes).

7. ANAPHASE II Sister chromatids pulled apart from each other Move to opposite ends.

8. TELOPHASE II Nuclear Envelope forms Spindle fibers break Cytokinesis occurs  4 haploid gametes.

H APLOID CELLS DEVELOP INTO MATURE GAMETES = G AMETOGENESIS G AMETES DIFFER BETWEEN MALE AND FEMALE Sperm become motile Really small, basically only contributing DNA to zygote. 4 sperm produced Much larger, contributes organelles, DNA, and cytoplasm, molecular building blocks, nutrients to zygote. 1 egg and 3 polar bodies produced (polar bodies get broken down)