Module 6 Cellular control

Question 1

Types of gene mutation

Answer 1

• Mutation is a change in the sequence of base pairs in DNA- leads to an altered polypeptide. Mutations are spontaneous and continuous.
• Mutagens – increase probability of mutations. Ionising radiation – breaks DNA strands. Deaminating chemicals – convert DNA bases. Methyl/ ethyl groups – incorrect base pairing. Viruses insert viral DNA. Normally have little effect.
• Insertion – addition of a nucleotide. Frameshift mutations. Change whole amino acid sequence.
• Deletion – nucleotide randomly deleted from sequence – frameshift mutation.
• Substitution – base randomly swapped – only change one amino acid. Silent- no change of amino acid as code is degenerate. Missense – alters single amino acid. Nonsense – premature stop codon. Polypeptide chain incomplete.
• Beneficial – altered characteristic beneficial. E.g., decreased production of melanin.
• Harmful – genetic diseases – loss of function of proteins.
• Neutral – no advantage or disadvantage.

Question 2

Lac Operons

Answer 2

• Operons- found in prokaryotes. Group of genes controlled by the same promoter.
• Lac operon – production of lactase which is an inducible enzyme. Only synthesized when lactase is present.
• Structure – Promoter for regulator gene, lac I (lac repressor protein – 2 binding sites for operator and lactase), promoter, operator, lac z (lactase), lac y (permease), lac A (transacetylase).
• When lactase absent – regulator gene transcribed, and lac repressor protein produced. Lac repressor protein binds to operator region. RNA polymerase can’t bind to promoter region. Lactase enzyme not synthesized.
• When lactase present – uptake of lactose into bacterial cell. Lactase binds to second binding site of repressor protein. Can’t bind to operator site. RNA polymerase binds to promoter region. Transcription takes place. mRNA translated and lactase produced.

Question 3

Regulatory Mechanisms

Answer 3

• Transcription factors – found in eukaryotes. Bind to promoter region and allow/prevent transcription. Change shape of region. Allow/ inhibit RNNA polymerase from binding.
• Post transcriptional – introns and exons both transcribed. Removal of introns after transcription from pre-mRNA through splicing. Exons fused to form mature mRNA.
• Post translational – activation of proteins by cAMP. cAMP derived from ATP and action of adenyl cyclase. Activates protein kinases. Protein kinase A- inactive precursor enzyme. Activates glycogen phosphorylase in muscles – exposes active site.

Question 4

Body Plans

Answer 4

• Homeobox – DNA sequence that codes for a protein transcription factor. Attaches to DNA and regulates transcription of genes.
• Similar in animals, plants and fungi and highly conserved. Maintained by natural selection. All code for amino acid sequences -bind to DNA – same shape. Mutations not viable- not favourable in natural selection.
• Form basic pattern of body – polarity and segmentation.
• Hox genes – subset of homeobox genes. Embryonic body regions. 4 hox clusters – on different chromosomes. Linear order- reflected in order of regions of the body they affect.

Question 5

Mitosis and Apoptosis

Answer 5

• Mitosis – cell division resulting in two genetically identical cells. Growth, cell replacement and tissue repair.
• Apoptosis – programmed cell death. DNA becomes denser. Nuclear envelope breaks down and chromatin condenses. Vesicles containing hydrolytic enzymes form. Phagocytes engulf and digest cells.
• Importance- key mechanisms in development of body form. Cells that are no longer needed can be destroyed. Separation of fingers and toes.
• Controls – mitosis is controlled by proto-oncogenes which stimulate cell division and tumour-suppressor genes that reduce cell division and stimulate apoptosis in cells that have been genetically damaged.
• Checkpoints- regulate cell cycle by cyclins and cyclins dependent kinases. Cyclins are regulators and CDKs are catalysts that control progression of the cell cycle.
• Stimuli – mitosis and apoptosis can respond to external and internal cell stimuli. Cell stress can lead to apoptosis. External stimuli such as UV light, viruses and bacteria, cytokines, hormones and growth factors. Internal stimuli such as DNA damage, cyclin D and RNA decay.