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) Mitotic phase 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 Regulation 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 Apoptosis – preprogrammed cell death; examples: hand and feet development, leaf fall 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