KEY CONCEPT Cells have distinct phases of growth, reproduction, and normal functions.

Formation of new cells by cell division All organisms undergo cell

division for three main reasons. Growth and development Repair Reproduction

Most eukaryotic cells use mitosis and cytokinesis for growth, development, repair and asexual reproduction. Bacteria use binary fission for

asexual reproduction Mitosis is never used for

sexual reproduction, that requires meiosis.

Cells divide at different rates.

The rate of cell division varies with the need for those types of cells.

• Some cells are unlikely to divide (G0).

Chromosomes condense at the start of mitosis. DNA wraps around proteins (histones) that condense it.

DNA doublehelix

DNA andhistones

Chromatin SupercoiledDNA

DNA plus proteins is called chromatin.

• One half of a duplicated chromosome is a chromatid.

• Sister chromatids are held together at the centromere.

• Telomeres protect DNA and do not include genes.

Condensed, duplicated chromosome

chromatid

telomere

centromere

telomere

The cell cycle has four main stages.

The cell cycle is a regular pattern of growth, DNA replication, and cell division.

The main stages of the cell cycle are gap 1, synthesis, gap 2, and mitotic phase.

– Gap 1 (G1): cell growth and normal functions

– DNA synthesis (S): copies DNA

– Gap 2 (G2): additional growth

– Mitotic phase(M): includes division of the cell nucleus (mitosis) and division of the cell cytoplasm (cytokinesis)

There are three different checkpoints that can stop mitosis.– G1 Checkpoint: The

checkpoint decides whether S phase will happen. S phase will only happen if the cell is healthy and large enough.

– G2 Checkpoint: Checks that DNA replication happened ok.

– M Phase Checkpoint: Checks that mitosis has occurred properly.

Parent cell

centrioles

spindle fibers

centrosome

nucleus withDNA

Mitosis and cytokinesis produce two genetically identical daughter cells. Interphase prepares

the cell to divide. During interphase,

the DNA is duplicated.

Mitosis divides the cell’s nucleus in four phases.– During prophase, chromosomes condense and

spindle fibers form.

Mitosis divides the cell’s nucleus in four phases.– During metaphase, chromosomes line up in the

middle of the cell.

Mitosis divides the cell’s nucleus in four phases.– During anaphase, sister chromatids separate to

opposite sides of the cell.

• Mitosis divides the cell’s nucleus in four phases.– During telophase, the new nuclei form and

chromosomes begin to uncoil.

Cytokinesis differs in animal and plant cells.– In animal cells, the

membrane pinches closed.

– In plant cells, a cell plate forms.

Binary fission is similar in function to mitosis. Asexual reproduction is the creation of offspring

from a single parent. Binary fission produces two daughter cells genetically

identical to the parent cell. Binary fission occurs in

prokaryotes.

parent cell

DNA duplicates

cell begins to divide

daughter cells

Environment determines what form of reproduction is most advantageous.

– Asexual reproduction is an advantage in consistently favorable conditions.

– Sexual reproduction is an advantage in changing conditions.

Some eukaryotes reproduce through mitosis. Budding forms a new organism from a small

projection growing on the surface of the parent.

bud

Hydra

Yeast

Fragmentation is the splitting of the parent into pieces that each grow into a new organism.

• Vegetative reproduction forms a new plant from the modification of a stem or underground structure on the parent plant.

You have body cells and gametes. Body cells are also called somatic cells. Germ cells develop into gametes.

Germ cells are located in the ovaries and testes. Gametes are sex cells: egg and sperm. Gametes have DNA that can be passed to offspring.

body cells sex cells (sperm) sex cells (egg)

• Your body cells have 23 pairs of chromosomes.– Homologous pairs of

chromosomes have the same structure.

– For each homologous pair, one chromosome comes from each parent.

• Chromosome pairs 1-22 are autosomes.

• Sex chromosomes, X and Y, determine gender in mammals.

Your cells have autosomes and sex chromosomes.

Body cells are diploid; gametes are haploid. Fertilization between egg and sperm occurs in

sexual reproduction. Diploid (2n) cells have two copies of every

chromosome. Body cells are diploid. Half the chromosomes come from each parent.

• Haploid (n) cells have one copy of every chromosome.

– Gametes are haploid.– Gametes have 22 autosomes and 1 sex chromosome.

• Chromosome number must be maintained in animals.• Many plants have more than two copies of each

chromosome.• Mitosis and meiosis are types of nuclear division that

make different types of cells.• Mitosis makes

more diploid cells.

• Meiosis makes haploid cells from diploid cells.

– Meiosis occurs in sex cells. – Meiosis produces gametes.