Chromosomes and Cell Reproduction Chapter 6 Section 1

Structure of Chromosomes

Chromosome Structure

Homologous Chromosomes Each human somatic (any cell other than sperm or egg) cell normally has two copies of 23 different chromosomes, for a total of 46 chromosomes. Each of the 23 pairs of chromosomes consists of two homologous chromosomes Chromosomes that are similar in size, shape & genetic content

Diploid & Haploid Diploid Somatic cells Two sets of chromosomes 2n = 46 Haploid Gametes One set of chromosomes n = 23

Sex Chromosomes Autosomes are chromosomes that are not directly involved in determining the sex (gender) of an individual. Sex chromosomes, one of the 23 pairs of chromosomes in humans, contain genes that will determine the sex of the individual.

Karyotype Photo of the chromosomes in a dividing cell that shows the chromosomes arranged by size.

The Cell Cycle Chapter 6 Section 2

The Cell Cycle The cell cycle is a repeating sequence of cellular growth & division during the life of an organism. A cell spends 90% of its time in the first three phases (interphase) of the cell cycle. A cell will enter that last two phases of the cell cycle only if it is about to divide.

Interphase 1. G 1 Phase: Grows rapidly & carries out its routine functions. 2. S Phase: DNA is copied 3. G 2 Phase: Nucleus prepares to divide 4. Mitosis: Nucleus of a cell is divided into two nuclei 5. Cytokinesis: Cytoplasm divides

The Cell Cycle (pg. 126 fig. 7)

Control of the Cell Cycle Three principal checkpoints G 1 G 2 Mitosis If control is lost, cancer occurs

Mitosis & Cytokinesis Chapter 6 Section 3

Prophase Chromosomes coil up & become visible Nuclear envelope dissolves Spindles form

Metaphase Chromosomes move to the center of the cell & line up along the equator Spindle fibers link the chromatids of each chromosome to opposite poles

Anaphase Centromeres divide Chromatids (pairs of chromosomes) separate and move toward opposite poles as the spindle fibers attached to them shorten

Telophase Nuclear envelope forms around the chromosomes at each pole. Chromosomes uncoil Spindle dissolves Spindle fibers break down & disappear

Cytokinesis Cytoplasm of the cell is divided in half Cell membrane grows to enclose each cell, forming two separate cells

End Result of Mitosis & Cytokinesis Two genetically identical daughter cells each with its own nucleus wit identical chromosomes. The Cell Cycle Animated

Mitosis

Meiosis and Sexual Reproduction Chapter 7 Section 1

Meiosis Meiosis is a form of cell division that halves the number of chromosomes when forming specialized reproductive cells. Meiosis involves two divisions of the nucleus Meiosis I Meiosis II

Prophase I Chromosomes become visible Nuclear envelope breaks down Crossing-over occurs When portions of a chromatid on one homologous chromosome are broken & exchanged w/ the corresponding chromatid portions of the other homologous chromosome.

Metaphase I Pairs of homologous chromosomes move to the equator of the cell.

Anaphase I Homologous chromosomes move to opposite poles of the cell. Chromatids do not separate at their centromeres—each chromosome is still composed of two chromatids. The gentic material, however, has recombined.

Telophase I & Cytokinesis Chromosomes gather at the poles of the cell Cytoplasm divides

Prophase II A new spindle forms around the chromosomes

Metaphase II Chromosomes line up at the equator

Anaphase II Centromeres divide Chromatids move to opposite poles of the cell

Telophase II & Cytokinesis A nuclear envelope forms around each set of chromosomes. The cytoplasm divides

Meiosis & Genetic Variation The result of meiosis is four genetically different haploid cells. Three mechanisms make key contributions to this genetic variation: Independent assortment Crossing-over Random fertilization

Independent Assortment Independent assortment is the random distribution of homologous chromosomes during meiosis. Each of the 23 pairs of chromosomes segregates (separates) independently. Thus, 2 23 (about 8 million) gametes with different gene combinations can be produced from one original cell.