CHROMOSOMES AND CELL REPRODUCTION CHAPTER 6 CHROMOSOMES AND CELL REPRODUCTION

I. CHROMOSOMES A. Formation of New Cells 1. Prokaryotic Cells -Reproduce by BINARY FISSION -Create a genetic duplicate

2. Eukaryotic Cells -In a resting cell, DNA exists as thin, uncoiled strands known as CHROMATIN, and can be found in the Nucleus of the cell. -During cell division the DNA will REPLICATE and begin to coil around special proteins called HISTONES

-The DNA/HISTONE structure is together referred to as a CHROMATID -CHROMOSOMES consist of two chromatids or SISTER CHROMATIDS, that join together. -The parts, original and copy are attached by a CENTROMERE

DNA When a molecule of DNA is being used to direct a cell’s activities, areas containing specific active genes are extended.

II CHROMOSOME NUMBERS A. Sets of Chromosomes - Every species has a characteristic number of chromosomes in each body or somatic cell - Numbers vary among species: Humans = 46 or 23 pairs Homologous Chromosomes- Chromosomes that are similar in size, shape and genetic contents. They are pairs of chromosomes containing genes that code for the same traits.

- Cells that contain both chromosomes of a homologous pair are said to be DIPLOID (2n) these are somatic or body cells - Cells with one chromosome of a pair HAPLOID (n) these are gametes or sex cells.

B. Sex Chromosomes -These are the chromosomes that determine the sex of an individual. (1 pair) -All other chromosomes are referred to as AUTOSOMES (22 pair) -Sex chromosomes are labeled X , Y -In humans XX is female XY is male

C. Changes in Chromosome Number -Sometimes, due to various reasons, the number of chromosomes is altered. -Chromosome number can be checked using a number of methods… Chorion villi Sample Amniocentesis Karyotype

Prenatal-Testing Karyotype- is how chromosomes are checked. Of the 23 pairs of chromosomes in human somatic cells, 22 pairs are called autosomes. One of the 23 pairs are involved in determining the sex(gender) of an individual. Also known as sex chromosomes.

Together known as INTERPHASE-Longest phase of the cycle III. THE CELL CYCLE -Defined as a repeating sequence of cellular growth and division during an organisms life. A. The five phases of the cell cycle… First Growth (G1) Synthesis (S) Second Growth (G2) Mitosis Cytokinesis Together known as INTERPHASE-Longest phase of the cycle

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G1(first growth)- Cell grows and carries out normal. functions G1(first growth)- Cell grows and carries out normal functions. Cells that do not divide stay in this phase. S phase:(synthesis)- DNA replicates and chromosomes are made. G2:(second growth)- Final preparations for division are made Mitsosis- Division of the chromosomes into two separate, equal numbered sides Cytokinesis- Cytoplasm divides

B. Control of the Cell Cycle -Cell cycle has key “checkpoints” that can trigger or delay the following phases. -Checkpoints are controlled by enzymes. G1 Checkpoint- decides if cell will divide G2 Checkpoint- allows for the checking of DNA and proceeds to mitosis Mitosis Checkpoint- signals end of mitosis and the start of G1 phase.

Cell size is the main factor that leads to cell division.

C. Steps of Mitosis- PMAT Broken into four stages 1) PROPHASE: -The nuclear membrane and the nucleolus begins to breakdown -In animal cells the centrioles develop and move away from each other

-SPINDLE FIBERS, strands of protein connect the chromosomes and the centrioles. - A spindle fiber is a specialized form of microtubule.

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http://bio.rutgers.edu/~gb101/lab2_mitosis/graphics/aavideo.mov

2) METAPHASE: -Centrioles reach the opposite ends of the cell - The fibers move the chromosomes to the center (equator) of the cell. -The chromosomes are now lined up.

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3) ANAPHASE: -The sister chromatids that make up the chromosomes are pulled apart at the centromeres by the fibers. -The spindle fibers pull each chromatid strand toward the centrioles at opposite ends of the cell.

4) TELOPHASE: -Each end of the cell now has a complete set of chromosomes. -Chromatids unwind and return to DNA strands – Chromatin -Nucleus and Nucleolus reform

-The spindle fibers/centrioles disappear -The nuclear membrane begins to reform -The nucleolus reforms

D. CYTOKINESIS: Begins when mitosis ends. -Cytoplasm is divided in half between the two new daughter cells -Animal cells: form a protein belt that creates a “cleavage furrow” -Plant cells: create a “cell plate” that becomes the new cell wall

Cleavage furrow Cell plate

END RESULTS Two new cells smaller in size than the original -same # of chromosomes