The Cell Cycle

Why do cells divide? For growth, repair, and reproduction

When do cells divide? Most limiting factor in size is the size of the cell membrane. A cell has to stay small so that all the things that need to go in and out don’t have too far to travel. The surface area to volume ratio must be small so cells stay tiny and divide instead of growing too big.

How Do Cells Divide? Cell cycle - sequence of phases in the life cycle of the cell

Interphase 3 parts: G1, S and G2 G1 - cell growth; make new organelles S – DNA replication G2 – short; organelles for cell division are produced

Mitosis

What is Mitosis? The process whereby body cells (somatic) are made by cell division Asexual

How does it work? 4 Stages

1st - Prophase Chromosomes become visible Centrioles go to opposite sides Spindle fibers form Nuclear membrane breaks down

-2- Metaphase- Chromosomes line up along the equator in pairs -3- Anaphase- Chromosomes move to opposite ends of the cell -4- Telophase- a cleavage furrow forms and… each cell now has exactly the same number of chromosomes!

Cytokinesis Cytoplasm pinches in half

Mitosis Results in an exact duplicate of the same cell. There is no change.

Interphase

Prophase

Metaphase

Anaphase

Telophase

MEIOSIS Chapter 11

So, What is Meiosis? The process whereby gametes or, sex cells are made by cell division Examples are sperm & eggs only!

How does it work? Prophase II Metaphase II Anaphase II Telophase II 8 Stages Prophase I (includes chromosomes crossing over and exchange) Metaphase I Anaphase I Telophase I Prophase II Metaphase II Anaphase II Telophase II

The same things occur with chromosomes except the number of chromosomes is halved

Meiosis results in cells that have only 23 chromosomes in humans Men make 4 sperm cells Women make 1 egg and 3 polar bodies (which are excreted as waste products

Why is meiosis so important? It provides for genetic variation crossing over occurs in prophase I and II shuffling of chromosomes (or genetic information)

Mitosis Meiosis Body cells/ somatic cells 4 Stages 1 division Results in 2 daughter cells Asexual Growth 2n - diploid Sex cells / gametes 8 stages 2 divisions Results in 4 sperm or 1 egg & 3 polar bodies Sexual reproduction Crossing over – prophase I 1n - haploid