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Definitions:
Gene: Section of DNA that codes for a specific protein.
Allele: An alternative form of a gene
DNA: A double-helix nucleic acid which carries genetic information.
Chromosome: A highly coiled superstructure of DNA.
Transcription: Making mRNA from DNA
Translation: Making protein from mRNA
DNA Structure:
-DNA is a double-helix polymer made up of monomers known as nucleotides. The two strands of the double helix are known as antiparallel. This means they are orientated in opposite directions to each other.
-Nucleotides are composed of a sugar, a phosphate group and one of four nitrogenous bases: Adenine, Thymine, Guanine & cytosine.
-Bases on either side of the double helix form the rungs of the 'twisted ladder' by forming hydrogen bonds between complementary bases.
-Adenine forms two hydrogen bonds with Thymine, whilst Cytosine forms three hydrogen bonds with Guanine. You could therefore say that GC pairing is stronger than AT pairing.
DNA Diagram:
DNA
Transcription (nucleus)
mRNA
mRNA:
Single-stranded, Uracil not Thymine
mRNA moves out of nucleus to RER
(Where ribosomes are found)
Translation (ribosome)
Protein
Translation:
mRNA read in bases three at a time per 1 amino acid made. tRNA reads mRNA and brings amino acids to make polypeptide chain
DNA Structure vs Function:
Sugar-phosphate backbone negative charge, protects bases which face inwards.
DNA is very long, can store lots of information as genes which code for proteins.
Hydrogen bonds between bases are weak, easily broken, but strong in large numbers. Allows transcription bubble to be opened/DNA to be easily replicated (by DNA Helicase)
Flexible over long distances, rigid over short distances allows it to exist as a stable, coiled structure.
Complementary base pairing is always a great phrase to use in exam answers. Remember that the idea of complementarity is centred around shape.
Transcription (DNA to mRNA):
-RNA Polymerase unwinds the double helix, forming a transcription bubble, exposing the bases.
-RNAP then synthesises a strand of mRNA (integrating uracil into its single-strand rather than thymine.
-The mRNA (messenger RNA) then leaves the nucleus via the nuclear pore and travels to the RER (rough endoplasmic reticulum)
The RER contains many ribosomes. The mRNA enters the ribosome during translation
Properties of the genetic code:
Universal: The same rules for nucleic acids apply across nature. Transcription, translation and triplet codons.
Degenerate: More than one triplet codon codes for an amino acid. This means that if one base mutates to another base, the mutation can be regarded as 'silent' because the same amino acid will be coded for.
Non-overlapping: Code read in triplets, means that genes are kept separate and a single base can only form part of one codon.
Translation (mRNA to polypeptide/protein):
-Ribosome made of rRNA, ribosomal proteins and tRNA. The tRNA contains anticodons which bind to triplet codons on mRNA.
-Many different tRNA molecules (each bringing with it an amino acid) read the code and generate a polypeptide by joining amino acids together.
-Translation is terminated when a STOP codon is reached.
DNA replication:
-DNA Helicase unwinds the double-helix and leaves two exposed single strands of DNA.
-At this region (replication fork) free-floating nucleotides attach to the exposed bases.
-New strands of DNA are polymerised using DNA polymerase.
-DNA phosphate backbone is sealed by DNA Ligase (phosphodiester linkages)
-Two new strands of DNA have now been formed. However one strand is synthesised in short fragments at a time called Okazaki fragments.
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