What type of replication does DNA undergo?
- Semi-conservative replication: each new stand of DNA has one new strand and one old strand
What are the stages of the cell cycle?
- Interphase: G1 - cell content replication
S - DNA replication
G2 - Double check and repair - Mitosis: Cell division
What are the stages is DNA replication?
- Initiation
- Elongation
- Termination
What happens during initiation?
- Origin of replication is recognised
- Helicase unravels the DNAS double helix
- A primer and the enzyme primate start replication (DNA polymerase can only extend 3’ ends of pre-existing 5’ chains
What happens during elongation?
- Moving replication forks
- Helicase continues to unwind double helix
- DNA polymerase reads template strand from 3’ to 5’, and extends 3’ ends of the leading strand continuously, but extends the lagging strand discontinuously (as is moving away from the replication fork and must go back when the DNA unzips more) forming Okazaki fragments
- DNA lipase the joins the Okazaki fragments together
What happens during termination?
- Replication forks move towards each other from opposing directions
- DNA lipase joins strands when they meet (lagging to leading and vice versa)
- Chromosome number stays the same - one replicated chromosome consist of 2 sister chromatids
Compare and contrast mitosis and meiosis
Mitosis: Meoisis:
- For growth (somatic cells) - To produce gametes
- 1 round of replication - 1 round of replication
- 1 round of division - 2 rounds of division
- Forms 2 identical diploid - Forms 4 non-identical haploid
daughter cells daughter cells
What happens in each stage of mitosis?
- Prophase: spindle fibres appear; chromosomes condense; nuclear membrane breaks down
- Prometaphase: spindle fibres attach to chromosomes; chromosomes continue condensing
- Metaphase: chromosomes align at the equator
- Anaphase: centromeres divide; sister chromatids move to opposite poles pulled by the spindle fibres
- Telophase: nuclear membrane reforms; chromosomes decondense; spindle fibres disappear
- Cytokinesis: cytoplasm divides; parent cell becomes two daughter cells with identical genetic information
What happens in each stage of meiosis?
- Meiosis I:
~ Prophase I: spindle fibres appear; nuclear membrane breaks down; chromosomes condense
~ Metaphase I: chromosomes line up at the equator in homologous pairs (form a bivalent); crossing over occurs (chiasmata form between non-sister chromatids)
~ Anaphase I: each chromosome out of a homologous pair is pulled to opposite poles by the spindle fibres (independent assortment-random which way maternal and paternal chromosomes go)
~ Telophase I: nuclear membrane reforms; chromosomes decondense; spindle fibres disappear - Meiosis II: (Happens at 90o to meiosis I)
~ Prophase II: nuclear membrane breaks down; spindle fibres form; chromosomes condense
~ Metaphase II: chromosomes line up at the equator
~ Anaphase II: chromatids move to opposite poles pulled by the spindle fibres
~ Telophase II: nuclear membrane reforms; chromosomes decondense; spindle fibres disappear;
~ Cytokinesis: 4 haploid non-identical daughter cells are formed
How is genetic diversity increased in meiosis?
- Random assortment of chromosomes
- Crossing over of genetic material
Why are meiosis daughter cells haploid?
- Maintains constant chromosome number from generation to generation
Define genotype
- The genetic make-up of an individual (either as a whole or one specific gene locus)
Define phenotype
- All observable characteristics of an individual or the expressed trait as a result of the genetic makeup of one or more specific genetic locus/loci
What do environmental factors have an effect on?
- Directly on phenotype
- On genotype which then has an effect on the phenotype
What environmental factors have as effect on the genotype and phenotype?
- Radiation
- Mutagens
- Chemicals that affect cell growth
- Diet
- Lifestyle
Define gene
- A unit of hereditary
- A length of DNA on a chromosome that contains the code for a protein
Define allele
- An alternative form of a gene
- Each individual has two alleles for every gene (one maternal, one paternal) which can be the same or different
Define homozygous
- Both alleles are the same (can be homozygous dominant or homozygous recessive)
Define heterozygous
- Two alleles are different
Define hemizygous
- Only one allele for a gene eg on X chromosome (only in males)
Define dominance
- Allele in a heterozygote that determines the phenotype
- Expressed in the phenotype in both homozygous dominant and heterozygous individuals
Define recessive
- Allele in a heterozygote that does not determine the phenotype
- Only expressed in the phenotype in homozygous recessive individuals
Define codominace
- Both alleles in a heterozygote are expressed in the phenotype
Define complementation
- More than one gene is responsible for phenotype
- A child of two recessive parents can be unaffected if parents have different defective genes
- Eg Albinism
