DNA, genes and protein synthesis - A-Level Biology notes PDF

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Chapter 8: DNA, genes and protein synthesis 8.1 Genes and the genetic code Gene: a section of DNA that contains the coded information for making polypeptides and functional learning Locus: a gene located at a specific position on the DNA Genetic code – there must be a minimum of three bases that code for each amino acid because: - Only 20 different amino acids regularly occur in proteins - Each amino acid has its own code - Only 4 different bases ( TACG ) - Using one base only 4 amino acids will be created - Using a pair of bases only 16 amino acids will be created - Using three bases 64 amino acids will be created, which is enough to satisfy the 20 amino acids

Features of the genetic code: - A few amino acids are only coded by a single triplet - A degenerate code – as most amino acids are coded for by more than one triplet - Start and stop triplets - Code is non-overlapping meaning each base is read only once in the sequence - Code is universal meaning each triplet code is for the same amino acid in all organisms (indirect evidence of evolution)

8.2 DNA and chromosomes

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Prokaryotic cells: Circular, shorter DNA which is not associated with proteins therefore they don’t have chromosomes Eukaryotic cells: linear, longer DNA which is associated with proteins called histones that form chromosomes. The mitochondria and chloroplasts of eukaryotic cells have shorter DNA and don’t associate with proteins

Homologous chromosomes: - Sexually produce organisms are the result of the fertilisation of a sperm and an egg cell. - The maternal and paternal chromosomes contribute to a full set o Diploid – total number of chromosomes o Haploid – half the total number of chromosomes

Alleles: an alternative form of a gene that occurs at the same locus on homologous chromosomes - Any changes in the base sequence of a gene causes a mutation (a new allele of that gene). This results in production of a different polypeptide which may or may not be functional - Ex. If the mutated protein is an enzyme and it isn’t complementary to the substrate it will cause a problem

8.3 Structure of ribonucleic acid Transferring the coded information: - Sections of DNA are transcribed onto mRNA - mRNA is small enough to leave the nucleus into the cytoplasm where proteins are synthesised - A codon: the sequence of three bases on mRNA that code for an amino acid

Chapter 8: DNA, genes and protein synthesis -

A genome: a complete set of jeans in a cell A proteome: a full range of proteins produced by the genome.

mRNA: 1. The base sequence of mRNA is formed from DNA through transcription 2. mRNA leaves the nucleus through nuclear pores and enters the cytoplasm where it associates with ribosomes 3. mRNA possesses information in the form of codons 4. mRNA is the least stable (from DNA and tRNA) tRNA: - Small and folded into a clover-leaf shape - As the code is degenerate there must be as many tRNA molecules as there are coding triplets. Each tear and it is specific to one amino acid and has an anti-codon that specific to that amino acid - Manufactured in the nucleus but found throughout the soul - More stable than mRNA 8.4 Polypeptide synthesis – transcription 1. DNA helicase separates the two strands of DNA to expose the nucleotides in a specific region 2. One of the strands is used as a template strand to which free nucleotides from the pool, pair with the complimentary nucleotides on the strand. RNA polymerase joins the nucleotides together to form pre-mRNA (Adenine binds to Uracil) 3. Only about 12 base pairs on the DNA are exposed at a time throughout transcription 4. When RNA polymerase reaches a stop triplet it detaches and production of pre-mRNA is complete 5. In prokaryotic cells transcription results directly in mRNA. In eukaryotic cells pre-mRNA is spliced to form mRNA. DNA of a gene is made up of exons (code for proteins) and introns (don’t code for proteins). Introns are therefore removed and exons are joined together. 6. mRNA molecules are too large, so they have to diffuse through the nuclear pores 8.5 Polypeptide synthesis – translation 1. A ribosome becomes attached to the starting code on at one end of mRNA 2. The tRNA, carrying an amino acid, with the complimentary anticodon sequence moves to the ribosome and pairs up with the codon on the mRNA 3. Another tRNA molecule, carrying another amino acid, with a complimentary anticodon pairs with the next codon on the mRNA 4. Ribosome moves along mRNA bringing together 2 tRNA molecules together 5. 2 amino acid is on the tRNA are joined by a peptide bond using an enzyme and ATP which is hydrolysed to provide the required energy 6. Ribosome moves onto the third codon on the mRNA 7. The first tRNA molecule is released and is free to collect the same amino acid from the pool if needed 8. The process continues until the polypeptide chain is built and a stop codon is reached -

mRNA is determined by DNA line-up of tRNA is determined by mRNA...