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. 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 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. 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 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