OCR Biology A2 Unit 2 Module 1 Notes-Cellular Control


GENE: length of DNA that codes for one or more polypeptides.

POLYPEPTIDE: a polymer consisting of a chain of amino acid residues joined by peptide bonds.

GENOME: the entire DNA sequence of that organism. The human genome consists of about 3 billion nucleotide base pairs.

PROTEIN: large polypeptide-100 or more amino acids.

The genetic code: the sequence of bases on a gene (length of DNA) provides a code, it is a triplet code (3 bases codes for an amino acid), degenerate code, there are stop triplet codes, widespread but not universal.

Transcription: the gene unzips-dips into nucleolus, H bond break, activated RNA nucleotides bind, with H bonds, to their exposed complementary bases, catalysed by RNA polymerase, phosphates released, releases energy, mRNA complementary, copy of coding strand base sequence, mRNA released, passes through nuclear pore to ribosome.

Ribosomes: rRNA and proteins, 2 subunits, ribosomal groove.

tRNA: made in the nucleus, passes into cytoplasm, fold into hairpin shapes and have 3 exposed bases at one end where a particular amino acid can bind. At the other end of the molecule are 3 unpaired nucleotide bases, known as an anticodon.

How the polypeptide is assembled: 1. mRNA binds to a ribosome, 2 codons attached to small subunit and exposed to large subunit, AUG forms H bonds with tRNA, methionine, UAC. 2. Second tRNA binds to 2nd exposed codon. 3. Peptide bond forms between two amino acids, enzyme catalyses the reaction. 4. Ribosome moves along mRNA, reading next codon, 3rd tRNA brings another amino acid and peptide bond forms, 1st tRNA leaves and is able to collect and bring another of its amino acids. 5. Chain grows until a stop codon is reached (no corresponding tRNAs.)

MUTATION: a change in the amount of, or arrangement of, the genetic material in a cell. CHROMOSOME MUTATIONS: changes to parts of or whole chromosomes. DNA MUTATIONS: changes to genes due to a change in nucleotide base sequences.

Certain substances cause mutations: tar found in tobacco, UV light, X-rays and gamma rays. Mistakes may happen when DNA is replicating before nuclear division, by either mitosis or meiosis. Mitosis-somatic mutations-not passed on to offspring-may contribute to the ageing process or may lead to cancer. Meiosis-gamete formation-can be inherited (passes to offspring). Point mutations-substitutions, insertion/deletion mutations-causes a frameshift. Diseases resulting from DNA mutations: cystic fibrosis-deletion of an amino acid, sickle cell anaemia-substitution, protooncogenes-changed into oncogenes-substitution-switched on, cell division, Huntington's-repeat of triplet.

ALLELE: alternative version of a gene-still at the same locus on the chromosome and codes for the same polypeptide but the alteration to the DNA base sequence may alter the protein's structure.

Early humans in Africa almost certainly had dark skin. Melanin protected them from harmful affects of UV light, but could still make vitamin D, mutations producing paler skin would have burned and suffered from skin cancer.

OPERON: is a length of DNA, made up of structural genes and control sites. The structural genes code for proteins, such as enzymes. The control sites are the operator region and




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