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Biology A2 Flashcards
Created for the English GCE A2-level Edexcel 2008 Specification
Not Applicable

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Explain the nature of the genetic code:
- Triplet code; each amino acid coded for by three nucleotides/bases.
-Non-overlapping; each triplet is discrete, each base can only be used in one triplet.
-Degenerate; there are 64 possible combinations of bases however only 21 amino acids found in Eukaryotes. Therefore some must be coded for by more than one triplet.
-Only first two bases count in determining AA; if the last base is altered polypeptide remains unaltered so mutations have less effect.
Why is a gene able to code for a protein:
-A gene is a length of DNA/sequence of bases/chain of nucleotides.
-Gene consist of triplet code on the sense/coding strand.
-The code is degenerate, non-overlapping and has a start/stop codon.
-A specific sequence of amino acids codes for a protein.
Protein Synthesis:
-Sequence of bases is the code.
-Polypeptide strands separate by Hydrogen bonds breaking.
-Free RNA nucleotides complementary base pair with the anti-sense strand which acts as a template.
-mRNA attaches to the ribosome on the rough endoplasmic reticulum.
-tRNA molecule carries an amino acid to the ribosome.
-Anticodons of the tRNA complementary base pair to the codons on the mRNA (translation), which code for a specific amino acid.
-These amino acids join by condensation reaction / formation of peptide bonds.
-DNA helicase causes hydrogen bonds to break and the polypeptide strands to separate.
-One DNA strand is used as a template to form mRNA from free RNA nucleotides.
-These complementary base pair with the template strand (antisense strand)/non coding strand.
-Hydrogen bonds form between bases with the aid of RNA-Polymerase.
Post transcriptional changes/splicing:
-Genes have exons (coding regions) and introns, (non-coding regions).
-The Gene is transcribed into mRNA.
-Introns are then cut out leaving only exons.
-Exons are spliced tgether to make functional mRNA; these can be spliced in many ways so several types of mRNA will be produced and then translated into various proteins.
-One gene can form several related proteins.
-A specific amino acid attaches to a tRNA molecule.
-The anticodon on tRNA attaches to codon on the mRNA.
-Two tRNA molecules will be held in the ribosome at any one time.
-Peptide bonds form between adjacent amino acids with the help of peptidyl transferase.
-The process continues until the stop codon.
-Polypeptide is released into the rough endoplasmic reticulum.
Why is only 1% of DNA transcribed:
-Only some genes are transcribed as only one strand is transcribed.
-Different proteins/enzymes are required by different cells.
-Most DNA consists of introns (non-coding) DNA.
-There are repeat sequences of DNA.
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