F215 Module 1: Genes, Protein Synthesis and Mutations By Ms Cullen

Terminology nucleotide base gene polypeptide genome basic unit of DNA (made up of a pentose sugar, base & phosphate group) pyramidine or purines (A, C, T and G) a length of DNA that codes for 1 or more polypeptides a chain of amino acids joined by peptide bonds The entire DNA sequence of an organism

What is a gene? A gene is a length of DNA. Genes code for polypeptides; – structural proteins such as collagen and keratin, – haemoglobin – immunoglobulins (antibodies) – cell surface receptors – antigens – actin and myosin in muscle cells – tubulin proteins in cytoskeleton – ehannel proteins – electron carriers – enzymes

DNA is made up of two polynucleotide chains, joined together at the bases by weak hydrogen bonds. This structure is then twisted into a helix. It is known as the double helix. The bases pair in a particular way: Adenine with Thymine Cytosine with Guanine

If one half of a DNA strand had the base sequence: GATCGGTACGTTA What would the other half have?

This is the Watson – Crick model of DNA discovered in 1953

DNA Replication The double helix is unwound and the base pairs are separated by the enzyme DNA helicase. The hydrogen bonds between the bases are broken and the 2 strands separate. Each strand of the double helix acts as a template for the opposite strand. Deoxyribonucleotide monomers bond to the template. Bases pairing as normal (A & T, C & G). The deoxyribonucleotides are polymerised, making two double helices, each with one old strand (parent), and one new strand (daughter). This is semi-conservative replication.

The Genetic Code A sequence of bases on a strand of DNA provides a code for the construction of a polypeptide. This code is a triplet; 3 bases in a sequence code for an amino acid. There are 64 (4 3 ) triplet possibilities, but only 20 amino acids to be coded for, so more than adequate! We call this code degenerate. Some triplets do not code for an amino acid, but code for a STOP at the end of a polypeptide chain. Although the code is found in many different organisms, it is not universal.

Transcription Genes are found in chromosomes in nucleus, but where are proteins formed? A copy of the genetic code must therefore pass through a pore of the nuclear envelope and move into the cytoplasm. This copy is known as messenger RNA (mRNA)

RNA – Ribonucleic Acid RNA is found in the cytoplasm, ribosomes and nucleolus of a cell. It is different to DNA as its sugar is ribose instead of deoxyribose and it contains the base uracil (U) instead of thymine (T). RNA is single stranded.

mRNA – Messanger RNA

Transcription 1.The length of DNA ‘unzips’ to become a template. 2.RNA nucleotides pair up and bond (temporary H bonds) with complimentary bases. This is catalysed by enzyme RNA polymerase. 3.The sugar-phosphate groups of the RNA bond to form back bone of the single- stranded mRNA. 4.The single-stranded mRNA, a copy of the DNA will leave the nucleus and move into the cytoplasm.

Transcription of a gene

Complete Qs P.105 of OCR A2 textbook

Translation The second stage of protein synthesis, where amino acids assemble into a polypeptide chain. The sequence of the amino acids is determined by the triplets of bases, codons, on the mRNA. This occurs at ribosomes, which are either free in the cytoplasm or attached to the rough endoplasmic reticulum.

Ribosomes Ribosomes are formed in the nucleus of eukaryote cells from ribosomal RNA (rRNA). rRNA is single-stranded but folded into a complex series of shapes. mRNA fits between 2 subunits and the ribosome then moves along the mRNA. The ribosome reads the code and assembles the amino acids in the correct order. This forms the primary structure of a protein.

tRNA – transfer RNA Is single-stranded but has a complex 3D shape as a result of intramolecular hydrogen bonding. Transfer RNA ‘reads’ the genetic code. This process is called translation.

Transfer RNA (tRNA) tRNA is made in the nucleus and passes into the cytoplasm. At one end of the tRNA there are 3 exposed bases where amino acids can bind. On the other end there are 3 unpaired bases called an anticodon. Each anticodon will bind temporarily with a complimentary codon on the mRNA.

Transfer RNA

Translation of a length of mRNA at a ribosome and assembly of a polypeptide

Protein Synthesis DNA and RNA are involved in protein synthesis.

Questions: ATA AGA TTG CCC 1.How many amino acids does this length of DNA code for? 2.Write down the sequence of amino acids that this piece of DNA codes for. 3.What will be the base sequence on the mRNA which is made during the transcription of this DNA? 4.Using your answers to Qs 2 and 3, work out the anticodons of the tRNA molecule which will carry the amino acids to this mRNA – tyrosine - asparagine

These statements contain common errors, explain why each one is wrong and give a correct version: 1.‘The sequence of bases in a DNA molecule determines which amino acids will be made during protein synthesis’ 2.‘The amino acids in a DNA molecule determine what kind of protein will be made in the cell’ 3.‘The 4 bases in DNA are adenosine, cysteine, thiamine and guanine’ 4.‘During transcription, a complimentary mRNA molecule comes and lies against an unzipped part of a DNA molecule’

Mutations

The processes of replication, transcription and translation are very carefully controlled, but occasionally they can go wrong. A random, unpredictable change in DNA, or damage to a DNA molecule is known as a mutation. There are 3 kinds of mutation

Mutations 1.Substitution: One base is substituted with another. This does not usually have an effect as the code for DNA is degenerate ie each amino acid is coded for by more than one triplet. eg GAA and GAG code for same amino acid leucine. These are also called point mutations. 2.Deletion: Involves the loss of a base pair from the DNA molecule. This makes a huge difference! The code is read as triplets, so if a pair is missing the whole sequence is read differently. This is called a frame shift.

Mutations 3. Insertion: This is when a new base pair is added. Like deletion, this causes a frame shift and has a big effect on the protein which is made. Look at examples at top of P.109 in OCR A2 Biology textbook.

Mutations Each type of mutation can form a different amino acid sequence (primary structure) in the protein the DNA is coding for. This can then result in the secondary or tertiary structure of the protein being different. This usually causes the protein’s function to be disrupted. Usually this is harmful, but occasionally it is beneficial. Can you think of an example? Amazingly 99% of mutations have no effect on the phenotype of the organism.

Mutations Read Pages in OCR Biology A2 textbook: Explain what mutations are. What substances may cause mutations? Examples of disease caused by mutations. Explain how mutations can have neutral effects. Give examples of harmful and beneficial mutations.