Chapter 17

Transcription and Translation

Dr. Joseph Silver

from these three chapterswe 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 needto 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. 17.11)- add a 5 end cap and 3 end poly A tail

-mRNA modification (RNA splicing by splicesomes)

-introns = non coding segments (see fig. 17.11)-exons = coding segments

-ribozymes = RNA acting as an enzyme

eukaryotes have 3 RNA polymerases

I – makes rRNAII – makes mRNAIII – 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 transcription5) 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 protein5) A site - holds next AA to be added to protein

6) E site – tRNA minus its AA released from ribosome

Show video

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 sameproteins

the genetic code

DNA has the genesmRNA has the genetic code as codons

tRNAs have the anticodons

See fig. 17.24

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 CT always attaches to G

the order of these bases in DNA are your genes

when a gene is opened up and copiedit is necessary that the RNAs

are different from DNAor else the system would not work

you would teat RNA and DNA as the same item

soDNA has A, T, C, G (genes)

butRNA has A, U, C, G (codon)

in this way the shape of RNA is different from DNA

the genetic code (alphabet) has4 letters (ATCU)

the genetic code has 64 words

in the genetic codeeach word has 3 letters

AAA means somethingAAU means somethingAAG means something

each word in the genetic codeis message for an amino acid

tRNAs bring amino acids to the ribosomethe amino acid links up with the mRNAone amino acid is linked to the previous

amino aciduntil a long chain is formed

at the end of the messagethe chain of amino acids are released

and they then change their shapeto become a tertiary structure

in the correct shape to function

sometimes the processof copying DNA is incorrect

a mistake is made

also errors take place due toUV light, x rays, chemicals,

there are many kinds of errors

point mutationbase substitution

additiondeletion

inversionstranslocation

all cells whether in a single celled prokaryote

or a large multicellular eukaryoteare exposed to substances called

mutagenswhich cause errors in DNA

known as mutations

some mutations cause an errorin one amino acid

butsome mutations cause the entire

protein from that point onto be incorrect