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

-CHROMOSOMES consist of two chromatids or SISTER CHROMATIDS, that join together.

-The parts, original and copy are attached by a CENTROMERE

-The DNA/HISTONE structure is together referred to as a CHROMATID

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

- 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.

A. Sets of Chromosomes

- 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 SampleAmniocentesis Karyotype

• 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.

Karyotype- is how chromosomes are checked.

Prenatal-Testing

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)MitosisCytokinesis

Together known as

INTERPHASE-Longest

phase of the cycle

4 min

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 divideG2 Checkpoint- allows for the checking of

DNA and proceeds to mitosisMitosis 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 stages1) 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.

1 min

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.

30 sec

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

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

END RESULTS