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Transcription and Translation of DNA
How does DNA transmit information within the cell?
PROTEINS!
How do we get from DNA to protein???
The central dogma of Molecular Biology--DNARNAProtein
What are proteins and RNA? (review)
What are proteins?
A bunch of amino acids joined together (peptides
and polypeptides)
There are 20 Amino Acids: I.e. serine, threonine,
leucine, valine, tryptophan…etc…
Examples of amino acids:
Threonine
Tryptophan
Valine
What is RNA?
Ribonucleic acid
Is single-stranded, but can fold back on itself
Ribose sugar (not deoxyribose like DNA)
Uracil is in place of thymine
Uracil:
RNA is used to get the message out of the nucleus
and into the cytoplasm where proteins are made
…So if there are 20 amino acids and only 4 nucleic
acids how does this work?
Need a code:
41 ? = 4
42 ? = 16
43 ? = 64!
This combination of 3 nucleic acids is called a codon
Codons: (you don’t have to memorize these)
Note that there is redundancy in the genetic code:
20 amino acids and 64 codons
1st step: from DNA to RNA
= TRANSCRIPTION
Objective is to make an accurate copy of a small
section of an organisms DNA
4 Steps:
1. Initiation (Start signal located)
2. Elongation (copying of DNA to RNA)
3. Termination (Stop signal located)
4. Processing (Final changes made)
Initiation:
Each gene of double stranded DNA has a sense
strand and an antisense strand. The sense strand
contains the “instructions” for protein synthesis.
On the sense strand is a nucleotide sequence
(called a promoter sequence) where RNA polymerase
can bind. (RNA polymerase makes RNA)
The promoter sequence has two specific regions.
One is rich in T and A nucleotides and is referred to as
a TATA box.
Blackboard
Elongation:
(very similar to DNA replication)
RNA polymerase opens the DNA one section at a
time
RNA polymerase works in the 5’3’ direction,
nucleotides added to 3’ OH group
Since RNA is single-stranded, no okazaki
fragments
Many RNA copies of the same gene can be made
MCB 0402
Termination:
RNA polymerase will continue along the DNA
strand until a terminator sequence is encountered.
After this, the RNA polymerase molecule separates
from the DNA strand AND the new RNA molecule
dissociates as well.
the RNA is now called mRNA for messanger RNA
Processing:
this occurs only in eukaryotes
Initial mRNA molecule is called pre-mRNA
1. 5’ end is “capped” with a modified G nucleotide.
This is called a 5’ cap
2. 3’ has a long string of A nucleotides added to it.
This is called a poly-A tail
These two things help protect the mRNA transcript
3. Splicing: eukaryotes have a bits of DNA that don’t
code for anything (called introns) in between bits of
DNA that do code for proteins (exons).
Introns must be removed before the mRNA leaves the
nucleus. This is done by a Spliceosome.