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The cell cycle -Biology unit 2
Terms in this set (15)
Cell division occurs in 2 stages
1) Nuclear division- nucleus divides=mitosis+ meiosis
2) Cell division- follows ^ = whole cell divides
-When cell divides the daughter cell must receive an exact copy of the genetic material from the parent.
-before divides the DNA must be replicated
Semi- conservative replication
1) 4 nucleotides+ bases (adenine,guanine, cytosine +thymine) must be present.
2) Both strands of DNA molecule- act as a template for attachment of these nucleotides.
3) Enzyme DNA polymerase needed (to catalyse)
4) Source of chemical energy required (to drive process)
Steps in semi-conservative replication
1) enzyme DNA helicase breaks the hydrogen bonds linking the base pairs of DNA.
2)The double helix separates into its 2 strands+ unwinds.
3) Each exposed polynucleotide strand then acts as a template to which complementary nucleotides are attracted. (free nucleotides are attracted to complementary bases e.g A-T, C-G)
4) Energy used to activate these nucleotides.
Steps in semi-conservative replication 2
5) activated nucleotides joined together by enzyme DNA polymerase- to form 'missing' polynucleotide strand
(joined by strong covalent bonds- forming the phosphate sugar backbone. A-T create H2 bond/ C-G create covalent)
6) The new DNA molecules contain 1 of the original DNA strands i.e built into each of the new DNA molecules.
Evidence for Semi-conservative replication: Meselson+ Stahls 1957
-They based there work on 3 facts:
1) All bases in DNA contain nitrogen
2) Nitrogen has 2 forms: lighter 14N and the isotope 15N - which is heavier
3) Bacteria then incorporate nitrogen from their growing medium into any new DNA that they make
1) E.coli bacteria- grown on nutrient medium- 15N they then passed through one cell division cycle in ordinary nitrogen 14N medium (50min at 36oc)
2) After they then passed through 2nd+3rd cell division cycle (generation) into ordinary nitrogen medium (14N)
3) At each stage the bacterial DNA was sampled, isolated and subjected to density gradient centrifugation in salt solution (could distinguish different DNA densities by centrifuging them)
Proven to be wrong:
-The complete parent DNA molecule acts as a template for the new daughter molecule, which is assembled from new nucleotides.
-The parent molecule is unchanged.
-New molecule made up f deoxyribose, phosphate + organic bases,
-Of 2 molecules present- one would be made entirely of new material and the other entirely original material.
-Mit results in an increase in number of identical cells for growth and repair.
- produces 2 daughter nuclei that have the same number of chromosomes as parent. (Genetic makeup of 2 daughter nuclei- identical to Parent) -except in mutation.
-Mito- always comes before interphase (not dividing)
Before Mitosis: Interphase- chromosomes invisible, DNA replicates
1) Prophase- chromosomes appear, nucleus disappears
2) Metaphase- chromosomes at equator, spindle forms
3) Anaphase- chromatids pulled to poles
4) Telophase- Chromatids at poles, nucleus reforms
AFTER Mitosis: Cytokinesis- cytoplasmic division
Importance of mitosis
-Growth- when 2 haploid cells ( 23chrom) fuse together to form diploid (number of whole cell 46) diploid has genetic mater to form new organism. Mito ensures cell grows to resemble parents.
-Differentiation- These cells change to give group specialised cells e.g epithelium- animals/ xylem-plants. These divide by mito to give tissues with identical cells which perform a function.
-Repair- cells damaged/die- new cells produced with identical structure+function. If not same wouldn't function properly.
The cell cycle
- Cells don't divide continuously, undergo cycle: 3 stages:
-Interphase- most of cycle (90%),(resting phase as no division takes place) Divided into 3 parts:
- First growth (G1)- when proteins (from which the cell organelles are synthesised) are produced.
-Synthesis (S)- when DNA is replicated-
- Second growth (G2)- when organelles grow+ divide+ energy stores are increased.
The cell cycle 2
2) Nuclear Division- when nucleus divides either into 2 (mitosis) or 4 (meiosis)
3) Cell Division- follows ^ process in which the whole cell divides in 2 (mitosis) or 4 (meiosis)
info about cell cycle
-Length of a complete cell cycle-varies in organisms e.g mammalian about 24 hours- about 90% interphase.
-The length also depends on conditions- can be as little as 8 mins in fly embryo cells with optimum temp, nutrients+ oxygen.
-Organism- some bacteria divide every 20 min, others every 10 hours.
-Cell type- some human liver cells taken longer than a year to complete a cycle, where as hair follicle cells take 18 hours.
-Caused by rapid cell growth.
-Result of damage to genes that regulate mitosis and the cell cycle- leads to uncontrolled cell growth- tumour develops+ expands
-Can develop in any organ most commonly found: in lungs, prostate gland (male) breast+ ovaries (fmale), large.intest, stomach, oesophagus+ pancreas.
-Treatment often involves blocking some part of the cell cycle- if disrupted, cancer growth ceases.
-Drugs like Chemotherapy disrupt the cycle by:
1) Preventing DNA from replicating e.g cisplatin
2) Inhibiting the metaphase stage of mitosis by interfering with spindle formation e.g vinca alkaloids.
-Problem is they also disrupt cell cycle of normal cells, but more effective against rapidly dividing cells+ suffer more damage.
-Hair producing cells also suffer damage (as they are also rapidly dividing cells)
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