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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.
DNA Replication:
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DNA Helicase unwinds the double stranded DNA by breaking the hydrogen bonds holding complementary base pairs together. Formation of the replication fork.
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A primer (short oligonucleotide sequence complementary to 3' end of single stranded DNA)
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DNA polymerase attaches to the 3' end of the single stranded DNA, and polymerises a new strand in the 5'3' direction.
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Free-floating nucleotides present in the nucleus. On the other strand, DNA replication is discontinuous, and therefore is synthesised in short sections called Okazaki fragments
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DNA Ligase seals the sugar phosphate backbone by forming phosphodiester bonds between nucleotides.
Transcription (making mRNA in the nucleus):
The purpose of transcription is to copy the information held in the DNA in the nucleus, and to move that information to the ribosome so it can be read and translated into a protein.
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RNA polymerase unwinds DNA and forms a transcription bubble, which contains regions of single stranded DNA.
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Free-floating nucleotides will bind to the exposed bases on the strand, which acts as a template
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RNA polymerase will join these nucleotides together to form a strand of mRNA, which contains the base Uracil instead of Thymine.
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mRNA is single stranded and is therefore suitable for export out of the nuclear pore. Here the mRNA will be localised to the RER or cytoplasmic ribosomes.
Translation (mRNA code read and a protein is made):
The purpose of translation sees the mRNA strand created in the transcription step being turned into a protein also known as a polypeptide- a polymer made out of amino acids joined by peptide bonds).
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mRNA is fed into the ribosome, which reads the mRNA three bases at a time (triplet codons).
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This 'reading' is done by tRNA molecules containing a complementary anticodon to the mRNA triplet codon.
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If complementary base pairing occurs, then the amino acid brought by the tRNA will remain and prepare to bind to the next amino acid.
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Translation is initiated by START codons and terminated by STOP codons. These are three bases which signal the ribosome to stop translating and dissociate from the ribosome.
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The newly formed polypeptide chain is often referred to as the 'nascent' polypeptide chain.
DNA, mRNA, tRNA, rRNA:
DNA: Double-helix, deoxyribose sugar, double stranded. Contains information in the form of genes
mRNA: Single stranded, moves information from nucleus to ribosome for translation. Uracil instead of Thymine.
tRNA: Regions of single and double-stranded RNA. Clover-leaf shape and contains an anticodon and a binding site for an amino acid.
rRNA: Component of ribosome. rRNA and ribosomal proteins make up the translational machinery.
Evidence for semi-conservative replication:
E-Coli grown in heavy isotopes of nitrogen (15N) will incorporate the isotope into their nitrogenous bases in the DNA. After many generations, almost all bacteria will have only 15N in their genomes.
The bacteria are then transferred to a regular 14N medium and are allowed to grow. By doing so, the bacteria incorporated the regular 14N into their DNA during replication.
-By centrifuging these mixtures out, scientists were able to see how much 15N and how much 14N were in each sample. What they found was the amount of 15N present decreases over each division. This hybrid amount of 15N decreases as it makes up a smaller fraction of the DNA with each replication.
14N
15N
15N band becomes fainter over sequential divisions.
Export + Processing (PTM):
Proteins can be modified following translation. They can be phosphorylated, methylated, or have sugar residues added (such as a mannose tag). These changes are called post translational modifications.
Phosphorylation
Carbohydrate added
Methylation (-CH3 added)
Mutation (see mutation 2.5 for more):
Changes made to the sequence of bases in either DNA or RNA are termed mutation. This can have consequences on protein folding, as the wrong amino acid incorporated into a polypeptide chain could cause the protein to fold completely different. If it were an enzyme, this could threaten the specificity of the shape of the active site.
Other consequences: Truncation (shortening) of protein, elongation, lack of folding altogether.
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