BIOL1003 Study Guide - Final Guide: Dna Polymerase, Ribonucleotide, Cellular Compartment
Transcription (RNA synthesis)
• In eukaryotes, transcription takes place in the nucleus, translation in the cytosol and
cytosolic organelles.
• In bacteria, both transcription and translation take place in the same cellular compartment.
Structure of RNA
• Similar to DNA:
o Sugar-phosphate backbone linked by phosphodiester bonds.
o Base sequence carries information.
• Different to DNA:
o Single stranded.
o Different sugar (ribose).
o One different base (uracil
instead of thymine).
Polymerisation
• DNA: Deoxy-ribonucleoside triphosphate (dNTP) – a nucleotide.
• RNA: Ribonucleoside triphosphate (rNTP) – a ribonucleotide.
• The directionality of laying down nucleotide monomers is 5' end
to 3' end.
Transcription (DNA-dependent RNA synthesis)
• Complementary base pairing used to produce an RNA strand that carries the same information
as the DNA template.
• Requires template DNA strand and ribonucleotide triphosphates (NTPs).
o New strand determined by complementary base pairing.
• DNA and RNA have chemically different ends, called 3' and 5' ends.
1. Initiation – transcription is initiated when RNA polymerase binds to a
specific DNA sequence called a promoter.
2. Elongation - addition of complementary rNTPs. Reaction proceeds to build
the new strand in the 5’ → 3’ direction (i.e. NTPs added to 3' OH).
3. Termination – at the terminator, RNA polymerase disengages from the
DNA site and disengages from the RNA strand.
Signal Selection and Amplification
• Up to 20,000 human genes all up.
• mRNA transcription – over 200,000 transcripts of ~ 10,000 genes
expressed in a cell.
• Single genes may be expressed at levels between 1-10,000 mRNA copies
per cell.
• mRNA degrades quickly, has to be replenished to maintain pool.
• mRNA reflects what the cell is doing.
The Genetic Code
• The rules used to convert RNA information into protein information.
o Combinations of 4 bases must specify 20 amino acids.
o 3 bases per word (codon) is the minimum number that will give 20 possibilities.
• Crick and Brenner demonstrated that the code was a triplet code.
o 3 bases = 1 codon, specifies 1 amino acid
Breaking the Genetic Code
• Marshall, Nirenberg and JH Matthaei:
find more resources at oneclass.com
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• 'The dependence of cell-free protein synthesis in E. coli upon naturally occurring or synthetic
template RNA'.
• Poly-U mRNA yielded a phenylalaline-only polypeptide. (uracil, uracil,
uracil….)
Translation
The Genetic Code
• Every protein has the start codon AUG.
• Stop codons = UAA, UAG and UGA.
• Features of the Genetic Code:
o Degenerate and redundant, but not ambiguous (not a single codon
that will code for two separate amino acid).
o Start and stop codons used in translation (unlike promoters and terminators in
transcription).
o Non-overlapping - 3 nucleotides and then 3 nucleotides, etc.
o Universal (almost)
o There are 3 possible reading frames.
Reading the Genetic Code
• Conversion of a base sequence to an amino acid sequence requires an adaptor molecule.
o Bases and amino acids are not complementary.
• The adapter molecule is a small RNA molecules called a transfer RNA (tRNA).
• The tRNA molecule is a translator in the sense that it can read a nucleic acid word (the mRNA
codon) and interpret it as a protein word (the amino acid).
• A tRNA molecule connects codons and amino acids.
• Hydrogen bonding between complementary base pairs makes the interaction between a
codon and tRNA anticodon specific.
Three Types of RNA Participate in Protein Synthesis
• mRNA - encodes a protein
• tRNA - adaptor molecules that convert a sequence of codons to a sequence of amino acids.
• rRNA - a component of the ribosome.
Protein Synthesis is Catalysed by the Ribosomes
• Ribosomes facilitate the specific coupling of tRNA anticodons with mRNA
codons during protein synthesis.
• The ribosome is made up of a large subunit and a small subunit.
• Each subunit consists of proteins and RNA molecules (called ribosomal
RNA, or rRNA).
• Ribosome structure depends on rRNA.
Protein Synthesis on the Ribosome
• Using information from a template mRNA.
• Amino acids are linked by covalent bonds.
• Protein grows in one direction only - amino acids added to C terminus.
The Ribosome has Two Binding Sites for tRNA Molecules
• The A (amino acyl) site - the next amino acid/tRNA waits for peptide bond
formation.
• The P (peptidyl) site - holds the growing protein attached to a tRNA.
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
In eukaryotes, transcription takes place in the nucleus, translation in the cytosol and cytosolic organelles. In bacteria, both transcription and translation take place in the same cellular compartment. Structure of rna: similar to dna, sugar-phosphate backbone linked by phosphodiester bonds, base sequence carries information, different to dna, single stranded, different sugar (ribose), one different base (uracil instead of thymine). Polymerisation: dna: deoxy-ribonucleoside triphosphate (dntp) a nucleotide, rna: ribonucleoside triphosphate (rntp) a ribonucleotide, the directionality of laying down nucleotide monomers is 5" end to 3" end. Initiation transcription is initiated when rna polymerase binds to a specific dna sequence called a promoter: elongation - addition of complementary rntps. Reaction proceeds to build the new strand in the 5" 3" direction (i. e. ntps added to 3" oh): termination at the terminator, rna polymerase disengages from the. Dna site and disengages from the rna strand. Breaking the genetic code: marshall, nirenberg and jh matthaei:
