Chapter 17 Transcription and Translation Dr. Joseph Silver

from these three chapters we will study the following topics - transcription - translation - alternative splicing - genetic code - mutations

The Central Dogma reception-transduction-gene(DNA)-RNA-protein to do this you need to understand transcription & translation

Transcription -the opening of one gene in a DNA strand (see fig. 17.9) - unzipping the double helix (RNA polymerase) -copying one of the DNA strands (template strand) - making pre-mRNA (see fig ) - add a 5 end cap and 3 end poly A tail -mRNA modification (RNA splicing by splicesomes) -introns = non coding segments (see fig ) -exons = coding segments -ribozymes = RNA acting as an enzyme

eukaryotes have 3 RNA polymerases I – makes rRNA II – makes mRNA III – makes tRNA and other RNAs

the process is very complex but can be described as follows 1) chemicals known as transcription factors bind to the DNA 2) RNA polymerase attaches to the transcription factors and 4) forms an initiation complex which begins the process of transcription 5) pre mRNA made 6) Pre mRNA spliced (5cap, 3 poly A tail, introns removed, exons joined)

Translation - using the codons of mRNA to make a protein - rRNAs form free and RER bound ribosomes - tRNAs each bring a specific amino acid to the ribosome - completed protein released from ribosome see fig. 17.4, 17.17c, & 17.18

the steps to protein production are as follows 1) formation of translation initiation complex (rRNA+ proteins) 2) assembly of ribosome about the initiation complex(rRNAs) 3) elongation factors stimulate synthesis of protein(translation) 4) P site - holds AA being added to protein 5) A site - holds next AA to be added to protein 6) E site – tRNA minus its AA released from ribosome

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free ribosomes – make proteins which are mainly used in the cytoplasm bound ribosomes – make proteins of the endomembrane system, ER, Golgi, lysosomes, vacuoles, nuclear membrane, plasma membrane, and proteins secreted out of the cell

polyribosomes = complex of ribosomes making the same proteins

the genetic code DNA has the genes mRNA has the genetic code as codons tRNAs have the anticodons

See fig it is a summary of the chapter

DNA is made up of a long string of 4 nitrogen bases adenine, thymine, cytosine, guanine A always attaches to C T always attaches to G the order of these bases in DNA are your genes

when a gene is opened up and copied it is necessary that the RNAs are different from DNA or else the system would not work you would teat RNA and DNA as the same item

so DNA has A, T, C, G (genes) but RNA has A, U, C, G (codon) in this way the shape of RNA is different from DNA

the genetic code (alphabet) has 4 letters (ATCU) the genetic code has 64 words in the genetic code each word has 3 letters

AAA means something AAU means something AAG means something

each word in the genetic code is message for an amino acid tRNAs bring amino acids to the ribosome the amino acid links up with the mRNA one amino acid is linked to the previous amino acid until a long chain is formed

at the end of the message the chain of amino acids are released and they then change their shape to become a tertiary structure in the correct shape to function

sometimes the process of copying DNA is incorrect a mistake is made also errors take place due to UV light, x rays, chemicals,

there are many kinds of errors point mutation base substitution addition deletion inversions translocation

all cells whether in a single celled prokaryote or a large multicellular eukaryote are exposed to substances called mutagens which cause errors in DNA known as mutations

some mutations cause an error in one amino acid but some mutations cause the entire protein from that point on to be incorrect