Overview
Transcription Detail
Another Transcription Animation
Part I: TRANSCRIPTION• the building of mRNA, tRNA, and rRNA (takes place in the NUCLEUS)
•Initiation:
•RNA polymerase binds to the gene’s promoter on the template
strand of DNA(a specific sequence of the DNA that acts as a “start
signal” for transcription)
•RNA polymerase unwinds and separates the two strands of the DNA.
•Elongation:
•RNA polymerase adds and then links complementary
RNA nucleotides as it “reads” the gene” on the
template strand.
•Termination:
•A “stop” signal on the DNA tells the RNA polymerase
to detach from the DNA and release the RNA molecule
DNA coding strand nucleotides (usually ~5,000 nucleotides):
ATT CGC ACC TAA
mRNA
UAA GCG UGG AUU
RNA Processing
Before leaving the nucleus……..
1. A cap (methyl-guanine or mG) is added by
enzymes to the starting end of the mRNA
molecule
2. A poly-A tail is added to the end of the
mRNA
3. The molecule is spliced. Introns are
removed (non-coding nucleotides) and
exons remain.
Translation
The assembly of a protein (occurs on the
ribosome in the cytoplasm)
Translation Detail
1. mRNA leaves the nucleus through the
nuclear pore and forms a functional
ribosome with two ribosomal subunits, and
a tRNA
2. Initiation: The mRNA “start” codon AUG is
oriented in a region of the ribosome called
the P site where the tRNA molecule carrying
methionine can bind to the start codon.
Translation- cont.
3. The codon in the area of the ribosome called the A site is
ready to receive the next tRNA.
4. Elongation: Both the A site and the P site are holding
tRNA molecules- each carrying a specific amino acid. A
peptide bond forms between the adjacent amino acids
5. The tRNA in the P site detaches and leaves its amino
acid behind
6. The tRNA in the A site moves to the P site. The tRNA
carrying the amino acid specified by the codon in the A
site arrives.
7. Termination: Steps 4-6 are repeated until a stop
codon is reached.
Translation animation
One more Translation animation
If a segment of DNA is:
TAC AAA GTA ACT
The mRNA strand would be:
AUG UUU CAU UGA
This would code for the following amino acids:
Met
Phe
His
Stop
Types of Mutations
•Point Mutationsa change of one or just a few nucleotides in a
gene
•Substitutionone nucleotide is replaced by another.
sometimes called a “missense” mutation
•Insertion –
an extra nucleotide is added.
•Deletiona nucleotide is omitted.
Frame shift mutations are caused by deletions and
insertions
Original Strand
AUG AAU GCG GAC UAA
Start - Asparagine - alanine –aspartate –stop
Ex. Deletion
AUG -AU GCG GAC UAA
becomes AUG AUG CGG ACU AA
Start -methionine -arginine- threonine-
Ex. Insertion
AUG
AAU
GCG
becomes AUG AAU
GGAC UAA
GCG GGA CUA A
Start- Asparagine- alanine - glycine- Leucine -