1 Central Dogma: Review of Transcription and Translation in bacteria

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3 Sense, antisense Compare the sense strand of the DNA to the mRNA. Note that mRNA synthesis will be 5’ to 3’ and antiparallel.

4 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.

5 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.

6 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.

7 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

8 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 1 st two bases in the codon.

9 The Genetic Code-2

10 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 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

11 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

12 tRNA: the decoder a.a. attaches here anticodon

13 Simultaneous transcription and translation No processing, no nucleus; mRNA already where the ribosomes are, so they get started quickly.