chapter 17 transcription and translation dr. joseph silver
TRANSCRIPT
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