Transcript File
Cellular Control revision lesson 1 Section 1 – 7 in textbook Key topics: - DNA Transcription Translation - Mutations - The Lac operon - Genes and body plans (Hox genes) - Apoptosis Structure of DNA – pg 104 Adenine = Thymine (2 Hydrogen bonds) Cytosine = Guanine (3 Hydrogen bonds) Deoxyribose sugar and phosphate backbone. Pyrimadines: Single ring. C and T Purines: Double ring. A and G The genetic code • A group of 3 bases codes for one amino acid. This is a base triplet • Only one strand of DNA codes for proteins = reference strand • Gene – a length of DNA that codes of one polypeptide. A A T Codes for… Leucine Protein Synthesis – Transcription –pg 105 1. 2. 3. 4. Hydrogen bonds between bases broken by DNA Helicase – DNA unzips Free nucleotides diffuse into position with exposed bases on reference strand, using complemetary base pairing. Condensation reactions catalysed by RNA polymerase creates mRNA. mRNA molecule breaks free from DNA and leaves the nucleus through a nuclear pore. Arrives in the cytoplasm and moves to a ribosome. Protein Synthesis – Translation – pg 106 • • 1. 2. 3. 4. 5. 6. tRNA molecules are used – have a series of 3 bases called an anti-codon In the cytoplasm, tRNA transferases load AAs onto specific tRNA molecules, depending on their anti-codon. In the ribosome, there is complementary base pairing between codons and anti-codons. Another tRNA molecule approaches and joins. A condensation reaction forms a peptide bond between the 2 AAs. The mRNA moves, and the first tRNA leaves Another amino acid is brought in. A chain of AA’s continues to be built up until a stop codon is reached on the mRNA. Chain of Amino acids (polypeptide) Petide bond Anti-codon Codon/ base triplet Ribosome http://www.youtube.com/watch?v=41_Ne5mS2ls Mutations – pg 108 • Mutation = an unpredictable change in the structure of DNA, or the structure or number of chromosomes. Types of mutations: Substitution – swapping one base for another. Often has no effect because DNA code is degenerate (one AA is coded for by more than one base triplet) Deletion – Loss of one base pair. Causes a frame shift, so all the following base triplets are disrupted. Insertion – The addition of a new pair of bases. Also causes a frame shift. Germline mutations – Mutation in gamete, so can cause genetic diseases and are passed on. Somatic mutations – Not inherited but can cause ageing and cancer. They are as a result of mutations in normal diploid cells. Sickle cell anaemia Causes: Mutation in Haemoglobin beta gene, where a T is substituted for an A (a point mutation on codon 6) Normally Gleucine is present, but Valine is instead. Tertiary structure of haemoglobin changed because Gleu is hydrophilic, but Val is hydrophobic. Symptoms: Some erythrocytes pulled out of bi-concave shape and become sickle-shaped. This means they cannot carry enough oxygen. Also, the sickle shaped erythrocytes can easily get stuck in blood capillaries and block them. Causes ‘sickle cell crisis’ The Lac Operon – pg 112 • • 1. 2. 3. 4. Genes responsible for lactose digestion and their regulation. Physical presence of lactose results in transcription of genes to produce lactose permease and Beta-galactosidase. A regulatory gene produces a repressor protein. When no lactose is present, it joins with the operator region. RNA polymerase joins to the promoter region. If the repressor protein is attached to the operator region, the RNA polymerase cannot move further along and transcript structural genes to produce enzymes for lactose digestion. If lactose is present, it binds with the repressor protein and causes a conformational change, so the repressor protein can’t bind with the operator region. RNA polymerase can travel along and the enzymes for lactose digestion can be produced. Produces repressor proteins RNA polymerase binds to promoter region Repressor protein binds to operator region http://www.youtube.com/watch?v=oBwtxdI1zvk Homeobox genes – pg 115 • Homeobox genes are genes whose activity switches a whole set of other genes on or off, affecting an organsim’s body plan (overall design of an organism’s body). • They are found in clusters called hox clusters • Most animals have very similar homeobox genes. • Genes are highly conserved (have not evolved much) • Code for production of transcription factors. These can bind to certain sections of DNA and cause it to be transcribed. • Thalidomide disrupted the homeobox genes in developing foetuses, so arms and legs did not develop properly. Drosophilia Fruit fly body plan pg 114 Maternal effect genes – determine embryo polarity (which end head, which end tail) Segmentation genes – specifiy polarity of each segment (which part of head is brain which part is mouth piece) Mutation Example 1: •Has a homeobox gene called Ubx, which prevents formation of wings in T3 ( a part of the body). •Mutation in both forms of Ubx = wings grow in T3 •Fruit fly has two sets of wings – cannot fly Mutation Example 1 •Antennapedia – where the antennae look more like legs http://www.youtube.com/watch?v=LFG-aLidT8s Apoptosis – pg 116 • • • 1. 2. 3. 4. Apoptosis – Programmed cell death involved in development. Eg. Tail of tadpole falls off as it changes into a frog. Metamorphosis – a major change in the structure of an organism as it changes from one stage of its life cycle to the next. In humans, apoptosis kills off tissue linking fingers to prevent webbness. Cytoskeleton broken down by enzymes. The cell shrinks, the membrane blebs and the nucleus starts to disintegrate. Chromatin condenses. Cell fragments made with plasma membranes and contain organelles. Cell fragments are ingested and digested by phagocytic cells eg. WBCs.