Cell Division

Cell Division

• What is cell division?

• Why do cells multiply rather than get larger to grow?

Why Cell Divide

• Two reasons why cells divide rather than increase in size: – Increased demand on DNA (DNA overload) – Trouble getting nutrients and wastes across

the membrane

What is Cell Division?

• Cell division – a cell divides into 2 daughter cells o Each daughter cell is same size of parent o Each daughter cell receives same DNA (genes)

Prokarytotes - cell is separated into 2 parts called binary fission

Eukaryotes • Mitosis – division of the nucleus• Cytokinesis – division of cytoplasm

• By cell membrane or cell plate (plants)

Chromosomes• Chromosomes – made up of DNA, which carries the cell coded

genetic information

• Every species has a specific number of chromosomes, e.g., humans = 46 o Diploid – two sets (2N) as in an adult o Haploid (1N) as in an egg or sperm

• Chromosomes are not visible in most cells except during cell division

• Well before cell division, chromosomes are replicated (copied) – Each chromosome consists of 2 identical sister chromatids – Each pair of chromatids are attached at the centromere

Chromosomes

The Cell Cycle• Interphase (90% of cycle)

– G1 phase~ growth– S phase~ synthesis of

DNA– G2 phase~ preparation for

cell division

• Mitotic phase– Mitosis~ nuclear division

• Cytokinesis~ cytoplasm division

Mitosis

Prophase

• Chromosomes visible (before it was thin threads called chromatin)

• Nucleoli disappear• Sister chromatids• Mitotic spindle forms• Centrosomes move

Prophase

Metaphase

• Centrosomes at opposite poles• Centromeres are aligned• Kinetochores of sister chromatids attached

to microtubules (spindle)– Kinetochores - the protein structure on

chromosomes where the spindle fibers attach during division to pull the chromosomes apart

Metaphase

Anaphase

• Paired centromeres separate; sister chromatids liberated

• Chromosomes move to opposite poles• Each pole now has a complete set of

chromosomes

Anaphase

Telophase

• Daughter nuclei form• Nuclear envelopes arise• Chromatin becomes less coiled• Two new nuclei complete mitosis

Telophase

Cytokinesis

• Cytoplasmic division• Animals:

cleavage furrow• Plants:

cell plate

Cell Cycle Regulation

Cyclin – proteins that regulate the timing of the cell cycle in eukaryotic cells

Cyclin-dependent kinases (CDKs) – bind to cyclins to start various activities that take place in the cell cycle

Cell Cycle RegulationA. Uncontrolled cell growth – cancer cells do not respond to the signals

that regulate growth of most cells • Causes of cancer are various –

DNA mutations that produce proteins including those that regulate the cell cycle o Carcinogens - tobacco, radiation exposure, o viruses

Control over cell cycle broken down o No longer respond to external regulators or o Fail to produce internal regulators to ensure orderly growth

B. Apoptosis – preprogrammed cell death; examples: hand and feet development, leaf fall

C. Stem cells – unspecialized cells (embryonic and adult) Embryonic – after fertilization but before specialization

Adult – in organism, cells that can develop into other types of cells (EX: nervous into brain cells)

Meiosis

• What is different about the chromosomes? – Homologous chromosomes – chromosome that has a

corresponding chromosome from the opposite-sex parent

– Synapsis – when homologous chromosomes form pairs that are bound together along their length to form tetrad of 4 chromosomes ** Crossing-over – exchange of material (genes) between chromosomes

Tetrads (Homologous Pairs)

Crossing Over

Meiosis

• Meiosis – process of reduction division in which the number of chromosomes per cell is cut in half (Haploid: 1N) through the separation of homologous chromosomes in a diploid cell (Diploid: 2N)

Meiosis I

Meiosis II

Gamete Formation

• Gamete - o Spermatogenesis – produces 4 sperm o Oogenesis – produces one egg and 3 polar bodies

Gamete Formation