Central Dogma:
Review of Transcription and
Translation in bacteria
http://cats.med.uvm.edu/cats_teachingmod/microbiology/courses/gene_regulation/images/dij.tc.elong1.jpg
Sense, antisense
Compare the sense strand of the DNA to the mRNA.
Note that mRNA synthesis will be 5’ to 3’ and antiparallel.
http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/S/SenseStrand.gif
The Process of Transcription-2
• RNA synthesis continues (Elongation), only one
DNA strand (template) is transcribed.
• RNA nucleotides, complementary to bases on DNA
strand, are connected to make mRNA
• Termination: must be a stop sign, right?
– In bacteria, hairpin loop followed by run of U’s in the
RNA. Of course, the DNA must code for complementary
bases and a run of A’s. See next. Most common. OR
– Termination factor “rho”. Enzyme. Forces RNA
polymerase off the DNA.
Termination of Transcription in Bacteria
The hairpin loop
destabilizes the
interactions
between the DNA,
mRNA, and
polymerase; U-A
basepairs are very
weak, and the
complex falls apart.
http://www.blc.arizona.edu/marty/411/Modules/Weaver/Chap6/Fig.0649ac.gif
Transcription in prokaryotes
• As mRNA is made, it is ready to use.
• Info from more than one gene is typically found on
one mRNA molecule.
• Simpler process than in eukaryotes
– no introns to remove
– no cap or poly-A tail
– no nuclear membrane to transport through
• Transcription is expensive: each NTP leaves behind
2 Pi; like spending 2 ATP for every base used.
The Genetic Code
• Four bases taken how many at a time? Need to code for 20
different amino acids.
– Each base = 1 amino acid: only 4
– Every 2 bases = 1 a.a.: 16 combinations, 4 short.
– Every 3 bases: 64 combinations, enough.
• Every 3 bases of RNA nucleotides: codon
– Each codon is complementary to 3 bases in one strand of
DNA
Properties of the Genetic Code
• Code is unambiguous: 1 codon always specifies only 1
amino acid.
• Code is degenerate: although unambiguous, an amino acid
can be coded for by more than one codon.
• Punctuated: certain codons specify “start” and “stop”.
• Universal: by viruses, both prokaryotic domains, and
eukaryotes (except for some protozoa, mitochondria).
• Ordered: similar codons specify the same amino acid; see
especially the 1st two bases in the codon.
The Genetic Code-2
http://www.biology.arizona.edu/molecular_bio/problem_sets/nucleic_acids/graphics/gencode.gif
Bacterial ribosomes
• Prokaryotic ribosomes are 70S; eukaryotic are 80S
– S is Svedberg unit, how fast a particle travels during
centrifugation. Affected by both mass and shape.
• Large subunit: 50 S
– 33 polypeptides, 5S RNA, 23 S RNA
• Small subunit: 30 S
– 21 polypeptides, 16S RNA
• Note that 30 + 50 is not 70
• Ribosome structure and differences between prokaryotes
and eukaryotes are important.
– rRNAs important in taxonomy to be discussed later
– Differences are the basis for success of many antibiotics
Translation
• Literally, information translated from language of
nucleotides to that of amino acids
• Ribosomes (large and small subunits), mRNA, tRNAs,
amino acids, and source of energy.
– And various protein factors
• Ribosomes attach to mRNA, tRNAs read codons, match
amino acid to codon and ribosome connects amino acids to
make proteins.
• mRNA has start codon AUG and stop codons.
• Look for animations on line
• http://www.ncc.gmu.edu/dna/ANIMPROT.htm;
http://www.stolaf.edu/people/giannini/flashanimat/molgenetics/translation.swf
tRNA: the decoder
a.a. attaches here
anticodon
http://www.designeduniverse.com/articles/Nobel_Prize/trna.jpg
Simultaneous transcription and translation
•No processing, no nucleus;
mRNA already where the
ribosomes are, so they get
started quickly.
http://opbs.okstate.edu/~petracek/Chapter%2027%20Figures/Fig%2027-30.GIF