cell division. what is cell division? why do cells multiply rather than get larger to grow?
TRANSCRIPT
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Cell Division
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Cell Division
• What is cell division?
• Why do cells multiply rather than get larger to grow?
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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
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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)
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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
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Chromosomes
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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
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Mitosis
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Prophase
• Chromosomes visible (before it was thin threads called chromatin)
• Nucleoli disappear• Sister chromatids• Mitotic spindle forms• Centrosomes move
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Prophase
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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
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Metaphase
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Anaphase
• Paired centromeres separate; sister chromatids liberated
• Chromosomes move to opposite poles• Each pole now has a complete set of
chromosomes
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Anaphase
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Telophase
• Daughter nuclei form• Nuclear envelopes arise• Chromatin becomes less coiled• Two new nuclei complete mitosis
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Telophase
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Cytokinesis
• Cytoplasmic division• Animals:
cleavage furrow• Plants:
cell plate
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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
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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)
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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
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Tetrads (Homologous Pairs)
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Crossing Over
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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)
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Meiosis I
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Meiosis II
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Gamete Formation
• Gamete - o Spermatogenesis – produces 4 sperm o Oogenesis – produces one egg and 3 polar bodies
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Gamete Formation