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7.1 DNA structure and replication

DNA structure

• Part of DNA supercoiling are structures called Nucleosomes.
• DNA structure gives a clue to the mechanism of DNA replication.
• Non-coding regions of DNA have other important functions, limited to regulators of gene expression, introns, telomeres and genes for tRNAs.

DNA replication (in prokaryotes only)

• DNA polymerase enzymes can only add nucleotides to the 3’ end of a primer.
• Continuous DNA replication occurs on the leading strand and discontinuous on the lagging strand.
• A complex group of enzymes do DNA replication including; helicase, DNA gyrase, single strand binding proteins, DNA primase and DNA polymerases I and III.

Crossing over

• DNA replication makes a second chromatid in each chromosome in interphase before meiosis.
• Crossing over exchanges pieces of DNA between non-sister homologous chromatids and forms new combinations of alleles on the chromosomes formed in meiosis.

7.2 Transcription

• The direction of Transcription is in a 5’ to 3’ direction as RNA polymerase adds the 5´ end (phosphate) of the free RNA nucleotide to the 3´ end of the growing mRNA molecule.
• Transcription is partly regulated by Nucleosomes in eukaryotes.
• Eukaryotes modify mRNA after transcription.
• Splicing of mRNA increases the number of different proteins an organism can produce.
• Gene expression is regulated by proteins that bind to specific base sequences in DNA. - eg. methylation
• Gene expression is affected by the environment of a cell and of an organism.

7.3 Translation

Three stages of translation

• Initiation is the assembly of the components (large and small ribosome subunits, mRNA and tRNA molecules) that carry out the process.
• Synthesis of the polypeptide (elongation) involves a repeated cycle on a ribosome where tRNA binds to the A (aminoacyl), P (peptidyl) and E (exit) sites in turn. Polypeptide molecule is produced.
• (examples of start and stop codons not needed)
• Termination of translation is followed by disassembly of the components.

Ribosomes

• Free ribosomes synthesize proteins for use primarily within the cell.
• Bound ribosomes synthesize proteins primarily for secretion or for use in lysosomes.
• Translation can occur immediately after transcription in prokaryotes due to the absence of a nuclear membrane.

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  The sequence and number of amino acids in the polypeptide is the primary structure.
• The secondary structure is the formation of alpha helices and beta pleated sheets stabilised by hydrogen bonding.
• The tertiary structure is the further folding of the polypeptide stabilised by interactions between R groups. (Polar and non-polar amino acids are relevant to the bonds formed between R groups.)
• The quaternary structure exists in proteins with more than one polypeptide chain. and may involve the binding of a prosthetic group to form a conjugated protein.