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AQA A-Level biology Summary Unit 4 - Genetic information, variation and relationships between organisms
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Paper 1 revision - biology4 - Genetic information, variation and
relationships between organisms
DNA, genes and chromosomes:
- Euk. DNA -> long, linear, associated w/ histones, tightly coiled into CHR
- Prok. DNA -> short, circular, not associated w/ histones
- Gene = sequence of DNA bases that code for AA seq. of a polypeptide/functional RNA
o Occupies a fixed position, a locus, on particular DNA molecules
- Nature of genetic code:
o Universal -> same triplet code can code for same AA in all living organisms
o Non-overlapping -> a codon is specific to one AA
o Degenerate -> same AA can be coded for by more than one codon
- Genome = complete set of genes in a cell
- Proteome = full range of proteins a cell is able to produce
- Allele = diff. version of same gene
- mRNA:
o act as template for translations
o sequence of bases on RNA determines sequence of AA in polypeptide chain
o straight chain molecule
- tRNA:
o carries an AA
o has anticodons -> complementary to mRNA codon
o each tRNA is specific to one AA
o single polynucleotide strand
folded -> 3-leaf clover shape
held together by H bonds
- similarities and differences betw mRNA and tRNA :
o Similarities -> both single stranded
o Differences:
mRNA = single helix, tRNA = clover shape
mRNA = longer, tRNA = shorter
mRNA = has no paired bases/no H bonds, tRNA has paired bases and H bonds holding bases
- Transcription:
o DNA unzipped by DNA helicase -> each strand acts as template
o H bonds broken
o Free-floating RNA nucleotides align next to comp. bases on template strand -> form H bonds and U
replaces T
o RNA polymerase joins adjacent nucleotides, by condensation-> forms phosphodiester bonds
o When RNA polymerase reaches stop codon, mRNA (prok cells)/pre-mRNA (euk cells) detaches from
DNA
o mRNA leaves nucleus via nuclear pore
- After leaving nucleus, pre-mRNA is spliced to remove introns and exons are joined together -> no introns in
mRNA of prok cells
- Translation:
o Ribosome attaches onto mRNA strand and a tRNA binds to codon
Anticodon on tRNA is comp. to mRNA codon, H bonds form, and leaves an AA when tRNA
detaches and move along to next codon
o Does same thing w/ next codon -> 2 AA join together by condensation, forming a peptide bond,
using energy from ATP
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