DNA, genes, protein synthesis

• Variants of a gene is called alleles.
• The locus is the position of the gene on a chromosome.
• A homozygous individual has two identical copies of an allele at one locus.
• A nucleotide has 3 parts: pentose sugar, phosphate group, nitrogenous base.
• The chains in DNA run anti-parallel to one another.
• RNA is single stranded polynucleotide and is smaller and less complex.
• DNA contains four bases (adenine, thymine, cytosine, guanine) while RNA contains five bases (uracil).
• In DNA, A pairs with T and C pairs with G. In RNA, U replaces T.
• Hydrogen bonding between complementary bases holds the double helix together.
• The sequence of bases determines the genetic information encoded by the molecule.
• mRNA is found in single strands and is made via transcription.
• mRNA carries the code for making specific amino acids into the cytoplasm where they are assembled into proteins.
• tRNA carries amino acids to ribosomes during translation.
• tRNA is single stranded but 'clover-leaf' shaped, held by hydrogen bonds.
• Universal: the same specific base triplets code for the same amino acids in all living things.
• Non-overlapping: each base is read once and is only part of one triplet.
• Start codon (AUG) - methionine
• Stop codons (UAA/UAG/UGA)
• pre-mRNA is made up of introns and exons.
• Introns are removed from pre-mRNA by RNA splicing to form mRNA.
• There are three types of RNA involved in transcription: messenger RNA (mRNA), transfer RNA (tRNA), and ribosomal RNA (rRNA).
• DNA helicase breaks hydrogen bonds between complementary bases, which is the process of unwinding the DNA helix.
• Each new strand serves as a template for the formation of another strand.
• DNA polymerase catalyses the joining of adjacent RNA free activated nucleotides to form phosphodiester bonds.
• Mutations can affect the splicing of pre-mRNA and so can lead to non-functional polypeptides being made.
• Only mutations that affect gametes can be inherited.
• Mutagens: factors that increase the rate of mutations.
• Mutagens: high energy sources, certain chemicals.
• Substitution mutation: one base is substituted for another and so this will only affect one codon.
• Silent mutation: a base is substituted but the amino acid that is coded for doesn't change.
• Mis-sense mutation: a base is substituted and this usually results in an incorrect amino acid in the polypeptide sequence, so this can affect the proteins function.
• Nonsense mutation: a stop codon could be generated causing the production of the polypeptide chain to be halted. The final protein would be significantly different and so unable to carry out its function.
• Deletion/ addition mutation: this will change all following codons after the point at which it occurs. Consequence is considerable as the amino acid sequence of the polypeptide produced is entirely different.
• Frame-shift: the reading frame shifts and so the gene is now read in the wrong three base groups and the genetic message is altered.
• Meiosis: A type of cell division that produces four haploid daughter cells from one diploid parent cell.
• Meiosis: in the first meiotic division homologous chromosomes separate. In second meiotic division the sister chromatids separate.
• As fertilisation is random, this increases genetic variation within a species.
• Non-disjunction: this happens in meiosis cell division where homologous chromosomes fail to separate.
• The attachment of an amino acid via an enzyme to tRNA requires ATP.
• Degenerate: more than one triplet codes for same amino acids.