Flashcards · 7402 · A Level Topic 8

The control of gene expression (A Level) flashcards

17 cards on A Level Topic 8, subtopics 3.8.1 to 3.8.4. Pick a subtopic or study them all.

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3.8.1 Alteration of the sequence of bases in DNA can alter the structure of proteins

Which gene mutations cause a frame shift?: Additions, deletions and duplications, unless the number of bases is a multiple of three. Substitutions and inversions do not change the reading frame.
Types of gene mutation: Addition, deletion, substitution, inversion, duplication and translocation of bases. Mutations arise spontaneously during DNA replication; the rate is increased by mutagenic agents such as ionising radiation and some chemicals.
Why may a base substitution not change the polypeptide?: The genetic code is degenerate, so the new triplet may code for the same amino acid. Even if the amino acid changes, it may not affect the tertiary structure or function of the protein.

3.8.2 Gene expression is controlled by a number of features

Totipotent and pluripotent cells: Totipotent: can divide and produce any type of body cell (early mammalian embryo, for a limited time). Pluripotent: found in embryos; can form many cell types and divide without limit.
Multipotent and unipotent cells: Both found in mature mammals. Multipotent: form a limited number of cell types. Unipotent: form one cell type, for example cardiomyocytes.
Induced pluripotent stem (iPS) cells: Made from adult somatic cells (such as skin cells) treated with protein transcription factors, which switch on genes that give pluripotency.
How does oestrogen stimulate transcription of a target gene?: Lipid-soluble oestrogen diffuses into the cell and binds a receptor in the cytoplasm. The hormone–receptor complex acts as a transcription factor: it enters the nucleus, binds a specific base sequence near the gene, and RNA polymerase can bind.
Epigenetics: methylation and acetylation: Heritable changes in gene function without changes to the base sequence. Increased DNA methylation and decreased histone acetylation both pack DNA more tightly, inhibiting transcription.
RNA interference (RNAi): Small RNA (siRNA) in a protein complex pairs with an mRNA of complementary base sequence; the mRNA is hydrolysed, so it is not translated. The gene is still transcribed.
Oncogenes and tumour suppressor genes in cancer: Oncogene: a mutated or over-expressed proto-oncogene that stimulates continuous division (hypomethylation can raise expression). Tumour suppressor gene: loses function, e.g. by mutation or hypermethylation, so division is no longer slowed.

3.8.3 Using genome projects

What do genome projects do?: They sequence the whole genome (the complete set of bases in the DNA) of an organism, using automated sequencing methods.
Why is the proteome easier to work out for prokaryotes than for eukaryotes?: Prokaryotes have simple genomes with little non-coding DNA, so the proteome can be worked out from the genome. In complex eukaryotes non-coding DNA and regulatory genes make it hard to translate the genome into the proteome.
Uses of determining the proteome of a pathogen: Identifying the antigens of a pathogen helps in developing vaccines.

3.8.4 Gene technologies allow the study and alteration of gene function allowing a better understanding of organism function and the design of new industrial and medical processes

PCR: the three temperature stages: Denature at about 95 °C (strands separate). Anneal at about 50–65 °C (primers bind). Extend at about 72 °C (Taq polymerase builds new strands). Each cycle doubles the DNA.
How is cDNA made, and why is it useful?: Reverse transcriptase makes single-stranded cDNA from mRNA; DNA polymerase makes the second strand. The mRNA has no introns, so bacteria can express the gene.
DNA probe: A short, single-stranded, labelled (radioactive or fluorescent) piece of DNA complementary to a target allele. It hybridises only where the complementary sequence is present.
Genetic fingerprinting: what VNTRs are and how fragments are separated: VNTRs are short sequences repeated in a row; the number of repeats varies between people. In gel electrophoresis DNA (negatively charged) moves to the positive electrode; shorter fragments move further.