Why do cells divide?

The Cell Cycle

Why do cells divide?

Repair Growth Reproduction

Some organisms reproduce by cell division (asexual)

Limit on size

Chromosomes Genetic material is composed of DNA Most of the time, DNA is in the form of

chromatin (unraveled; in use)

Chromosomes Before the cell divides,

chromatin is replicated and condensed to form chromosomes (humans have 46)

Each chromosome consists of 2 identical copies called sister chromatids joined by a centromere

The Cell Cycle

The cell cycle consists of 2 phases: Interphase

G1, S, G2 Mitotic Phase

Mitosis (4 stages) Cytokinesis

Interphase

Time in between cell divisions Interphase is broken into 3 sub-phases:

G1 – cell works and grows S – DNA (chromatin) duplicates G2 – cell prepares to divide

Mitotic Phase

Mitosis – Division of nucleus Prophase Metaphase Anaphase Telophase

Cytokinesis – Division of cytoplasm

The Mitotic Spindle

Microtubules that separate the sister chromatids

Grows from centrosomes

Chromosomes are separated into sister chromatids

Chromosomes form Spindle fibers attach to the

chromatids Chromosomes line up in the

middle of the cell Spindle elongates the cell by

pushing the poles apart Genetic material exists as

chromatin

Chromatids are separated and move towards the poles

Spindle disappears 2 nuclei reappear The cytoplasm splits,

forming 2 cells Nuclear envelope breaks

down Cell is growing & working;

genetic material replicates

Mitosis

Prophase Chromosomes form Nucleus breaks down Centrosomes move to poles Spindle fibers attach to the

chromosomes

Mitosis

Metaphase Chromosomes line up in the

middle of the cell

Mitosis

Anaphase Chromosomes are separated

into sister chromatids Spindle elongates the cell

Mitosis

Telophase Chromatids reach the poles Spindle disappears 2 nuclei reappear Cleavage furrow appears

Cytokinesis

Cytokinesis The cytoplasm

splits, forming 2 cells

Summary

The cell cycle begins with one cell The cell undergoes a series of steps:Interphase – Prophase – Metaphase –

Anaphase – Telophase – Cytokinesis 2 identical daughter cells have been

produced

Restriction Point

Cell Cycle Checkpoints

Restriction point (G1) Internal surveillance External surveillance

Density dependence Anchorage dependence

Cancer

Benign tumor Malignant tumor

Metastasis Treatment

Surgery Radiation therapy Chemotherapy

Meiosis

Diploid vs haploid

Cells with two sets of chromosomes are called diploid (2n)

Cells with one set of chromosomes are called haploid (n)

Homologous chromosomes

You inherit one chromosome of each type from each parent

Homologous chromosomes are the 2 matching chromosomes 44 autosomes 2 sex chromosomes

http://en.wikipedia.org/wiki/File:NHGRI_human_male_karyotype.png

Meiosis

The process by which 4 haploid gametes are produced, each with half the number of chromosomes as the diploid parent Sperm – male gamete Egg – female gamete

If a human body cell has 46 chromosomes, how many does a human gamete have?

Meiosis

Meiosis involves 2 divisions Meiosis I – splits homologous pairs Meiosis II splits sister chromatids

Major Events

Prophase I Crossing over

Anaphase I Homologous pairs

separate Anaphase II

Sister chromatids separate

Meiosis promotes genetic diversity

Produces 4 genetically different daughter cells

During prophase 1, homologous chromosomes can exchange pieces of genetic material to form totally new chromosomes (crossing over)

Errors in Meiosis

A nondisjunction occurs when chromosomes or chromatids fail to split

Activity: Mitosis vs Meiosis

Fill in the chart to compare the two types of cell division.