Genetic code: DNA & RNA Flashcards
(30 cards)
Nirenberg-Matthaei Poly-U 1961
- determined which RNA coded for which protein
- found a codon was 3 nucleotides
- deduction: in order for the 4 nucleotides to account for all 20 amino acids, a minimum of 3 base pairs are required
Degenerate code
multiple codons code for the same amino acid
neutral/silent mutation
mutation of nucleotide order/ substitution - but does not influence aa encoded
neutral amino acid substitution
mutation in different aa produced - but does not affect protein function
how many stop codons are there? codons coding to stops translation
3 - no aa is produced - UAG, UAA, and UGA
how many start codons?
1 - methionine made
3 types of RNA
mRNA - carries transcripts DNA from nucleus to cytoplasm for protein synthesis
tRNA - transfers aas to mRNA to build proteins
rRNA - ribosomal RNA - make up ribosomes w protein
Nucleoside
sugar(ribose) + nitrogenous base
Nucleotide
nucleoside + phosphate molecule
DNA vs RNA ribose structure
Deoxyribose - DNA - H instead of OH on 2nd prime (attached to 2nd carbon in ribose pentagon) = better at long term storage
Ribose RNA - OH = less stable - better for short time/ dynamic use
DNA polymerisation (chain making) phosphate structure?
New joining nucleotides with 3 phosphates lose 2 when joining to make phosphodiester bonds - much like ATP (adenosine triphosphate) gives out energy, NTP (nucleoside triphosphates) provides energy for DNA synthesis.
Properties of B-DNA (typical double helix)
- 10 base pairs per turn
- right handed helix (rotates in clockwise direction)
- minor and major grooves
- very -ve charge due to phosphate groups
2 flavours of nitrogenous bases
Purines: Adenine, Guanine (double carbon rings)
Pyrimidine: Thymine, Cytosine, Uracil (single carbon rings)
why is Thymine used in DNA over Uracil?
- U makes more stable U-U bonds in mutation under UV radiation - t-t bonds break easier- less likely to be transcribed
- U paired with A causes kink in double helix backbone
- Cytosine easily mutates (through deamination- hydrolysis) to U - if U bases also existed in DNA, wouldn’t be able to tell the correct base
dNTPs
deoxynucleotide triphosphates = single nucleotides with 3 phosphates (free nucleotides)
chromatin
material of chromosomes - consists of: DNA, RNA, Proteins
nucleus dna types in microscope images
euchromatin - light coloured, sparsely packed dna, 90% of genome, expressed genes
heterochromatin - dark coloured, tight packed - supercoiled, silent genes
each nucleus contains 2m of DNA = supercoiling to fit
nucleosome
one “bead” of wrapped DNA along strand around histones (linker DNA with histone 1 H1 spans between nucleosomes - space for restriction enzymes to bind)
per nucleosome:
8 histones = “octamers”
147 base pairs
DNA wound 1.6 wraps around histone protein core
supercoil to form cremation fibres 30um
chromatin remodelling factors
protein complexes that travel along DNA to alter histones:
exchange, remove histones
alter nuclear or histone structure
histone tails
stick out of nucleosomes to alter gene expression, can be modified with:
methyl groups, acetyl groups, lysine
histone methylation
decreased DNA expression - inhibits binding of transcription factors by making histone tails wrap around DNA - often methylation can be reversed
DNA methylation
alters gene expression through modification of DNA - linked to disease - irreversible
chromatin fibre
Nucleosomes fold up to form a 30-nanometer chromatin fiber, condense into loops to form protein scaffold,
facultative heterochromatin
- heterochromatin = tightly packed
- but still able to be expressed - switches between hetero. and euchromatin states e.g, through histone tail modification
