Biology 1202B Lecture Notes - Lecture 13: Start Codon, Tata Box, Messenger Rna

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2 Apr 2013
Lecture 13: Transcription DNA to RNA
Transcription in Eukaryotes vs. Prokaryotes
Eukaryote Transcription and processing occurs
in nucleus, before exporting mRNA into the
cytoplasm for translation. Each process occurs
separately because they are compartmentalized
by the nucleus.
Prokaryote Transcription and translation of a
gene can occur simultaneously because the
genetic material is not confined to a nucleus. All
the different enzymes that are needed are free
floating in the nucleoid space.
Transcription - DNA RNA
- The information in DNA is
transferred to RNA
- DNA is double stranded, but
RNA is single stranded.
Therefore in transcription only
one strand is read the template
- Template strand and mRNA are
antiparralel, but when
nucleotides are trasnlated back it is goes back to the template strand.
- RNA polymerase creates an RNA sequence complimentary to DNA template
strand. Remember, that RNA does not contain Thymine, but instead, Uracil:
A-U instead of A-T. All others bind as in DNA replication
- The single stranded RNA is called the messenger RNA (mRNA)
Synthesis always occurs on a 5’ to 3’ prime direction because 3 prime end has
the OH where nucleotides join
Codon: three letters of the nucleotide sequence (Triplets)
- RNA is read in triplets 3 RNA nucleotides can be read in the universal code
chart. Codons are read in the 5’ to 3’ direction
If there is a mutation in the genetic code, what could change the protein?
Certain genetic mutations alter how the body functions because of what proteins
Universal Codon Chart
- Deciphers how codons correlate to amino acids and which proteins they
- Characteristics of the genetic code include universality and redundancy.
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- Universal same codons specify the same amino acids in all living organism
but there are some exceptions. Codon chart does not work for human
mitochondria, plant chloroplasts, or yeast
- Redundancy There are 64 different possible codons 4 different bases, A, U G,
T, times 3 spots (4x4x4)
- However, there are only 20 amino acids. This is because only the first two
nucleotides determine the amino acid. (but there are some only specified by
a single codon)
- Using the chart to make sense of the mRNA to see the protein sequence
Codons we have to know:
- Start codon: AUG specifies the amino acid methionine, and is the first codon
translated into any mRNA in both prokaryotes and eukaryotes
- Stop Codons: UAA, UAG, UGA, do not code for any amino acid, which indicate
the ending of a polypeptide sequence.
3 Major Stages of Transcription
- Assembly of the transcription components on
the promoter
- RNA polymerase binds to the promoter,
unwinds the DNA in that region and begins
synthesizing an RNA molecule
- *Another RNA polymerase may begin initiation
as soon as there is room at the promoter
- RNA polymerase moves along the DNA,
unwinding it and adding new RNA nucleotides
one by one in a 5’ to 3’ direction, to form a
strand of pre-mRNA.
- Releasing pre-mRNA RNA polymerase
- Reassembly of dsDNA by recoiling itself to be
ready for another round of transcription
Note: Once polymerase has started transcription and moved out of the way of the
promoter another molecule of RNA polymerase may start creating another pre-RN-
Polymerase and Transcription
The promoter specifies where on the DNA transcription begins. This structure
differs between the different types of cells:
Prokaryotes: all genes have similar proteins so use same RNA polymerase for
- RNA polymerase recognizes the key DNA sequences on the promoter and binds to
begin transcription
Eukaryotes: have different promoters
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