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Summary OCR A/AS-Level Biology 2.3.3 DNA Coding
Revision summary of key knowledge from OCR A/AS-Level Biology 2.3.3 DNA Coding.
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DNA CodingRNA
Ribonucleic acid contains ribose as the pentose sugar
The pyrimidine base uracil replaces thymine
The polynucleotide chain is shorter and single-stranded
Transcription
A section of the double helix unwinds and unzips, catalysed by the enzyme helicase, and
the hydrogen bonds between bases break
The enzyme RNA polymerase binds to the start codon called the promoter region and
then moves in the 5’-3’ direction, binding RNA nucleotides to DNA nucleotides with
weak, temporary bonds, then stops at the stop codon called the terminator region
This occurs on one strand of DNA called the template strand
A length of RNA is produced that is complementary to the template strand and
therefore identical to the other strand, called the coding strand
The length of RNA is now messenger RNA (mRNA) and passes out of the nucleus
through nuclear pores and attaches to a ribosome
Translation
Transfer RNA (tRNA) is made in the nucleolus and passes into the cytoplasm
At one end there are three unpaired nucleotides that recognise and attach to a specific
amino acid
At the other end there is another triplet of bases (an anticodon) which is
complementary with and binds to a triplet of bases on the mRNA (a codon)
Sequence of codons determines the sequence of amino acids, which determines the
primary structure of the protein and hence determines which protein is made
So long as the primary structure is correct, the protein will fold correctly
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