17 Genes _ How they work Part 2 09

17 Genes _ How they work Part 2 09 - Biol 61- Genes &...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
Part 2 (Chapter 17) 3/2/09 Translation in Bacteria We’ve just discussed how prokaryotes and eukaryotes transcribe and process their mRNA but, of course, the next step for mRNA is to be translated into a functional protein by the ribosomes of the cell. Ribosomes are large protein-rRNA complexes that consist of a small ribosomal subunit and a large ribosomal subunit. Ribosomes translate the mRNA into a polypeptide, but the entire mRNA is actually not translated! The overall structure of a bacterial mRNA is as follows: 5’-----------[Shine-Dalgarno]------------AUG --------------------------UAG------------------3’ [ 5’ UTR ][start coding region stop][ 3’UTR ] The “start” codon – and it is virtually always an AUG that is the first codon translated in a polypeptide - is NOT at the very 5’ end of an mRNA. Instead, there is a region of the RNA polymer between the start codon and the 5’ end of the message that is referred to as the 5’ untranslated region , or 5’UTR . In addition, the “stop” codon that specifies the end of the polypeptide is not at the very 3’ end of the mRNA. After the stop codon comes what is referred to as the 3’untranslated region ( 3’UTR ). The translated region between the start codon and the stop codon is called the coding region (since it contains all the codons used to make the polypeptide). “Upstream” of, or “behind”, the start codon in the 5’UTR is a particular sequence in the RNA called the Shine-Dalgarno sequence . This S-D sequence helps position the small subunit of the ribosome onto the correct spot of the mRNA transcript to begin translation. A ribosome is made up of a small subunit and a large subunit. Both subunits are actually pretty big compared to most other kinds of enzymes, and they are made up of multiple polypeptides and rRNAs. The small ribosomal subunit finds the 5’ end of the bacterial mRNA message and gets into the right position when it finds the Shine- Dalgarno sequence – the small ribosomal subunit contains a rRNA with a region that is complementary to the Shine- Dalgarno sequence, and when these RNAs form complementary base pairs it helps to position the ribosome at the correct location to begin translation (while most RNA is found in the form of a single-stranded polymer, RNA can also form complementary base pairs with another single-stranded RNA or even with itself – this makes little regions of RNA double-helix). After the small ribosomal subunit positions itself, it calls in a specific tRNA which is positioned at a particular spot within the small ribosomal subunit . tRNAs are the molecules that actually decipher the genetic code . A tRNA is a small RNA polymer that can form many base pairs with itself, so it has a complicated 3-dimentional structure. There are at least 20 different tRNAs to accommodate the 20 amino acids, each of which contains a 3-ribonucleotide
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

This note was uploaded on 01/15/2011 for the course BIOL 61 taught by Professor Vierra during the Spring '08 term at Pacific.

Page1 / 5

17 Genes _ How they work Part 2 09 - Biol 61- Genes &...

This preview shows document pages 1 - 2. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online