I. The Cell Cycle

• There are three stages to the cell cycle:1. Interphase

-G1 (growth)-S (DNA Replication)-G2 (growth and addition of organelles)

2. Mitosis-Prophase-Metaphase-Anaphase-Telophase

3. Cytokinesis

I. The Cell Cycle• The Cell cycle

represents the time it takes for a cell to grow, reproduce, and divide

• Some cells go through the Cell Cycle much quicker than others

A. Interphase

• Interphase is the longest part of the cell’s life

• Some cells (muscle and nerve cells) remain in interphase permanenty

• During interphase, the cell experiences growth, replication of DNA, and duplication of the organelles

B. Mitosis

• Mitosis is the division of the cell’s nucleus

• It occurs in four phases:• Prophase• Metaphase• Anaphase• Telophase

1. Prophase• The chromatin changes

into a more mature form:

chromatin chromatid chromosomes

• The nuclear membrane will disappear

• The centrioles and spindle fibers will appear

2. Metaphase

• Chromosomes will line-up at the cell’s equator

(move to the middle)

3. Anaphase• The spindle fibers

pull the chromosomes apart

chromosomes chromatid

4. Telophase• The chromatid

unwind into chromatin

• The centrioles and spindle fibers disappear

• The nuclear membrane reappears

Telophase in an animal cell:

C. Cytokinesis

• Cytokinesis is the division of the cell membrane and cytoplasm

• In plant cells, a cell plate forms in between the new cells. The cell plate eventually forms a cell wall.

C. CytokinesisC. Cytokinesis

In plant cells a cell plate forms in between In plant cells a cell plate forms in between the new cells and will become the cell the new cells and will become the cell membrane. A cell wall then forms around membrane. A cell wall then forms around the cells.the cells.

III. Mistakes in Mitosis

• Cancer is a disease in which cells grow and divide uncontrollably, damaging parts of the body around them.

IV. Meiosis

• Meiosis is the formation of sex cells or gametes.

• Sex cells contain half the number of chromosomes that normal body cells have• normal cells in humans have 46• sex cells in humans have 23

• When two sex cells combine they form a zygote.

IV. Meiosis

• Meiosis is basically mitosis twice without replication after the initial mitosis.

• Meiosis has two divisions:• the first division reduces the number of

chromosomes (from 46 to 23)• the second division makes another copy of

the cell

IV. Meiosis

A. The Results of Meiosis

• In Females – one egg and three polar bodies form

• In Males – four sperm cells form

V. Genetic Inheritance

• The male sperm cell and the female egg cell join together to form a zygote

• The sperm contributes 23 chromosome and the egg contributes 23 SIMILAR chromosomes

• These similar chromosomes pair up and form homologous chromosomes

A. Karyotype

• A karyotype is a picture of one’s chromosomes

B. Homologous Chromosomes

• These chromosomes are sorted into similar pairs called homologous chromosomes

• The 23rd pair tells us if it is male or female• Male XY• Female XX

VI. Mistakes in Meiosis

• Nondisjunction is one of the most common errors that occurs in meiosis

• In nondisjunction, some chromosomes fail to split apart during anaphase

• This causes the resulting cells to have too many or too few chromosomes

VI. Mistakes in Meiosis

A. Trisomy 21

• Trisomy 21 occurs when a person has an extra 21st chromosome.

• Some characteristics include slow mental development, heart problems, small head, a distinct crease in the palm of the hand

One day old infant