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CH8 - Cell Reproduction

Why do cells reproduce?

•Growth

•Healing

•Maintaining our bodies

Chromosomes

• Structures inside the nucleus that contains genetic information.

• Chromosomes are made from long strands of DNA and proteins.

Chromosomes

• Histones – protein that DNA molecules wrap tightly around.

• Non histones – protein that controls specific regions of the DNA molecules

Parts of a Chromosome

• Chromatid- Identical strands of DNA, half of a chromosome.

• Centromere- contsricted area of a chromosome connecting both halves.

• Centromere also helps control the movement of the chromosomes during cell division.

Numbers of Chromosomes

• Classified into two types: sex chromosomes or autosomes

• Two copies of the same chromosome are called homolgous chromosomes. –They are the same shape,

size and carry the same gene function.

Numbers of Chromosomes

• Every organism has two homologous chromosomes. Each offspring recieves one from each parent.

• Humans have 23 pairs of chromosomes, 46 in all (22 pairs of homolgous chromosomes and one pair of sex chromosomes).

Haploid vs. Diploid

•Haploid- Cell that has one of each type of chromosome.

•Diploid- Cell whose chromosome occurs in pairs.

Mitosis

• The process in which the nucleus of a cell divides to form two identical nuclei.

Cell Cycles

•Cells do not live forever!

•The cell cycle is a series of stages a cell goes through before it divides.

Pre Mitosis - Interphase

• Most of the cell’s life is spent in Interphase.

• G1 phase - A period of growth and development.

• S phase – DNA is copied • G2 phase - A period of growth and

preparation for cell division.

Steps of Mitosis

1. Prophase

2.Metaphase

3.Anaphase

4.Telophase

Step 1 - Prophase

• Nucleus disappears. • Chromosomes thicken • Spindle fibers stretch across

the cell. –These are threadlike

structures that pull apart the chromosomes.

Step 2 - Metaphase

•Chromosomes line up along the center of the cell.

Spindle Fibers attach

to the centromere.

Step 3 - Anaphase

•The spindle fibers pull the chromosomes apart.

Chromosomes are pulled

to opposite sides of the cell.

Step 4 - Telophase

•Spindle fibers disappear.

•The nuclei reforms.

•Chromosomes unwind.

Cytokinesis

• The cell pinches along the middle and becomes two new cells.

• The area where the cell pinches to form a new cell is called the cleavage furrow. That same area in plants is referred to as a cell plate.

Interphase Prophase Metaphase

Anaphase Telophase Cytokinesis

Results of Mitosis

• One nucleus becomes two nuclei.

• After mitosis, a new nucleus with 46 chromosomes (23 pairs) is made.

Meiosis

• Number of chromosomes reduced by half.

• Involves two stages and results in the formation of four new cells.

• Each new cell having haploid

cells.

Meiosis

• Cells undergoing meiosis still go through G1, S, and G2 stages.

• Since cells divide twice in meiosis the end result will be 4 new cells

Interphase

• Interphase in meiosis is identical to interphase in mitosis. Chromosomes replicate

• Centrioles appear outside the nulceus

Prophase 1

• Nucleus disappears.

• Chromosomes thicken

• Spindle fibers stretch across the cell.

Metaphase 1

• Here is where the critical difference occurs between Metaphase I in meiosis and metaphase in mitosis.

• Pairs of chromosomes line up randomly along the metaphase plate.

• Spindle fibers attach to each chromosome

Anaphase 1

• During Anaphase I the spindle fibers contract, pulling the homologous pairs away from each other and toward each pole of the cell.

• Since the centromeres have not divided the chromatids remain attached

Telophase 1

• A cleavage furrow forms

• The cytoplasm compresses and divides the cell in half.

• Each new cell contains the haploid number of chromosomes.

• Meiosis I is often called reduction division.

Prophase II

• The nuclear membrane disappears.

• A new set of spindle fibers forms

• The chromosomes begin to move toward the equator of the cell.

Metaphase II

• All the chromosomes in the two cells align with the metaphase plate.

Anaphase II

• The chromatids seperate.

• The spindle fibers shorten, drawing the chromosomes toward each pole of the cell.

Telophase II

• The formation of the nuclear membrane (envelope) occurs.

• Cells then divide leaving a total of four daughter cells, each with half the total number of chromosomes as the original cell

Sperm

23 Chromosomes

Haploid

Egg

23 Chromosomes

Haploid

Baby

46 Chromosomes

Diploid

Mistakes With Meiosis

•Sometimes sex cells are made with the wrong number of chromosomes.

•These organisms may not grow normally.