- Gene- a length of DNA that codes for one or more polypeptides. Each gene occupies a locus on a chromosome.
- Polypeptide- polymer consisting of a chain of amino acid residues joined by peptide bonds.
- A triplet codes for an amino acid. It is a degenerate code. The genetic code is widespread but not universal.
- Proteins are assembled in the cytoplasm on ribosomes.
Transcription- the creation of a single stranded copy of the DNA coding strand. MRNA is created when a gene is 'switched on'.
1. The desired gene unwinds and unzips as helicase breaks the hydrogen bonds between bases.
2. RNA polymerase binds to the promoter region. This is found on the template strand (5'-3')
3. RNA polymerase moves along strand. A complementary mRNA strand is formed from free nucleotides.
4. When terminator region is reached DNA is no longer copied.
Transcriptional factor needed to initiate in eukaryotes. They form a pre-initiation complex with RNA polymerase.
DNA contains introns that don't code for proteins- spliced out by spliceosomes to leave exons.
Translation- the assembly of polypeptides at ribosomes.
Ribosomes: made up of two subunits and travels along the mRNA reading the code. Order of the code is important as it effects the primary and tertiary structure.
1. mRNA binds to a ribosome. Each mRNA has a codon, start codon is AUG.
2. Transfer RNA bonds to mRNA by its anticodons. tRNA carried corresponding amino acids.
3. A second amino acids is joined through tRNA to the first: peptide bond formed by enzymes.
4. Third amino acid added. The first tRNA is now free to move into the cytoplasm. Continues until stop codon reached. No tRNA that corresponds, chain released by a release factor that hydrolyzes the chain.
6. Chain undergoes post translation modifications: start codon removed, functional groups added, structural changes like disulfide bridges.
SiRNA cuts the mRNA strand, and attached to mRNA which prevents full polypeptide form.
Mutation- a change in the amount of, or arrangement of, the genetic material in a cell.
Chromosome mutation- changes to parts of or whole chromosomes. DNA mutations- changes to genes due to change in nucleotide base sequences.
Two main types of DNA mutations:
Point- one base pair replaced by another. (substitutions)
Insertion/ deletion- one or more nucleotide pairs inserted or deleted. Causes a frameshift.
Example of genetic diseases:
1.Cystic fibrosis- caused by deletion of a triplet of base pairs.
2. Sickle cell anaemia- point mutation on polypeptide chain for haemoglobin.
3. Huntingtons disease- too many repeat nucleotide triplets.
The lac operon
An operon- a length of DNA made up of structural genes and control sites; the control sites are the operator and the promoter and they don't code for polypeptides.
Apoptosis- the programmed death that occurs in multicellular organisms. Cells undergo about 50 mitotic divisions (Hayflick's constant) before apoptosis.
Necrosis- cell death that occurs after trauma and releases hydrolytic enzymes.
1. Enzymes break down the cell cytoskeleton. Cytoplasm becomes dense, organelles tightly packed. Cell surface membrane changes and 'blebs' form (bumps that protrude from the membrane).
2. Chromatin condenses and the nuclear envelope breaks; DNA breaks into fragments.
3. Cell breaks into vesicles that are taken up by phagocytosis- cellular debris disposed of without effecting other cells.
Phagocytosis- the endocytosis of large solid molecules into a cell.
Apoptosis controlled by many cell signals; include cytokines, hormones, nitric oxide. Nitric oxide induces apoptosis- makes inner mitochondrial membrane more permeable to H+, so proteins are released into cytosol, binding to the apoptosis inhibitor proteins. Apoptosis benefits: components reused, excess cells disposed of, hydrolytic enzymes and ineffective T lymphocytes destroyed, limb digits separated, cell production balanced.
Meiosis: a reduction division. The daughter cells have half the original number of chromosomes- haploid cells used for reproduction.
Prophase 1: Chromatin condenses and undergoes supercoiling- shorten and thicken. The chromosomes come together in their homologous pairs to form a bivalent- each pair consists of one paternal and one maternal chromosome. These chromatids attach at a chiasma, and may swap sections with one another (crossing over).Nucleolus disappears and nuclear envelope disintegrates. A spindle made of protein microtubules forms.
Metaphase 1: Bivalents line up across the equator of the spindle- attached at centromeres. Each member of the homologous pair is facing a different pole of the cell so the chromosomes can independently segregate when pulled apart.
Anaphase 1: Homologous chromosomes in each bivalent pulled by spindle fibres to opposite poles. The centromeres do not divide. Chiasmata separate and leave the crossed over legs with their new chromatid.
Telophase 1: Two new nuclear envelopes form and cells divide (cytokinesis). Chromosomes then uncoil.
Meiosis II is in a plane at right angles to meiosis 1.
Prophase II: If a nuclear envelope has reformed, it breaks down again. The nucleolus disappears, chromosomes condense and spindles form. Centrioles replicate and move to poles in the new two cells formed in meiosis 1.
Metaphase II: Chromosomes arrange themselves at the spindle equator. They are attached to the spindle fibres at the centromeres. The chromatids of each chromosome are randomly assorted.
Anaphase II: The centromeres divide and the chromatids are pulled to opposite poles by the spindle fibres. The chromatids randomly segregate.
Telophase II: Nuclear enveloped reform around the haploid daughter nuclei. In animals these two cells then divide to give four haploid cells.
Meiosis increases genetic variation by:
- Crossing over during prophase I shuffles alleles: non-sister chromatids wrap around each other at chiasmata. The chromosomes may break here and reform in the same bivalent.
- Genetic reassortment and subsequent segregation of maternal and paternal chromosomes during meiosis I: each gamete receives a different mixture of peternal and maternal genes.
- Genetic reassortment and segregation of chromatids at meiosis II: This is due to random assortment on the spindle equator at metaphase II. This effects the segregation at anaphase II.
- Random mutation: This may occur during interphase when DNA replicates.
- Genotype: Alleles present within cells of an individual, for a particular trait/ characteristic. If two alleles are the same in an organism, that particular gene is homozygous.
- Phenotype: Observable characteristics of an organism.
- Dominant: An allele which is always experessed in the phenotype whether or not the other allele is the same or different.
- Recessive: An allele which is only expressed when no dominant alleles are present.
- Co-dominant: Two alleles of the same gene are expressed in the phenotype of a heterozygote.
- Linkage: two or more genes on the same chromosome that are inherited together.
- Sex linkage: A characteristic resulting from a gene in one of the sex chromosomes.
- Epistasis: the interation of different gene loci so that one gene locus supresses the expression of another. You can have dominant or recessive epistasis.
Chi- squared test
The chi-squared test is used to test the null hypothesis.
The null hypothesis is based on the assumption that there is no significant difference between the observed and expected numbers sand any difference is due to chance.
If there is no significant difference then we accept the null hypothesis.
X^2= the sum of (observed numbers - expected numbers)^2 / expected numbers