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Lecture 9

BIOL 3113 Lecture Notes - Lecture 9: Ribosomal Rna, Eukaryote, Protein Production


Department
BIOL
Course Code
BIOL 3113
Professor
Barbara S
Lecture
9

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9 Transcription and Translation
The Central Dogma of molecular biology: The flow of genetic information in cells is
from DNA to RNA to protein
All cells from bacteria to humans express their genetic information by two-step process:
1. DNA transcribed into RNA
2. RNA translated into a polypeptide
Many identical RNA copies can be made from the same gene why?
Each RNA molecule can be translated to many identical protein molecules -
amplification
*Better strategy than using a DNA as a direct template (only1 or 2 copies of each gene
per cell)
*Each gene can be transcribed and translated with different efficiency - allows production
of different proteins in different quantities according to actual need
Expression of genes is regulated
Protein production is a three-step process in eucaryotes:
1. Transcription
Synthesis of an RNA strand complementary to one strand of the DNA double helix
2. Processing (only in eucaryotes)
Addition of a cap and a tail
Splicing of the RNA to remove intron sequences and form mRNA
3. Translation
Conversion of the mRNA code into an amino acid chain
requires:
*tRNA's
*amino acids
*ribosomes
RNA -ribonucleic acid
Composition
Sugar - ribose
Bases:
Purines - adenine and guanine
Pyrimidines - cytosine and uracil
Because RNA is single stranded - may fold into a variety of complex 3-D shapes,
stabilized by complementary base pairing
Ability of RNA to fold into a variety of complex 3-D shapes allows to carry variety of
functions in cells other than passing genetic information between DNA and proteins

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Functions
Messenger RNA (mRNA) - code for proteins
Transfer RNA (tRNA) -amino acid carriers & adaptors between mRNA and amino acids
Ribosomal RNA (rRNA) - parts of the ribosome
Catalytic structures
Primer in DNA replication
Genetic information in some viruses
Transcription - DNA-dependent RNA synthesis
Enzyme - RNA polymerase , large, multi-subunit enzyme
One type in procaryotes, three types in eucaryotes
RNA polymerase:
*catalyzes the addition of a 5'-ribonucleoside triphosphate to the 3'-end of the RNA-
transcript
*unwinds the DNA in front of itself as it advances along DNA double helix
*rewinds the DNA after it is transcribed
*does not require a primer
*less accurate than DNA polymerase
*several polymerases can work on the same DNA fragment
The RNA transcript is complementary and anti-parallel to the DNA template strand -->
the template and transcript have opposite polarity
To initiate transcription, the RNA polymerase must bind firmly to the DNA double helix:
binding occurs at promoter sites
requires other proteins (factors) to permit binding
Promoter:
*site on the DNA to which RNA polymerase attaches
*consists of a specific base sequence that is unique for each type of RNA polymerase
*establishes: 1. the orientation of the DNA polymerase on the DNA double helix, 2.
which of the two strands will be the template strand, 3.the start-site for transcription
In eucaryotes, initiation of transcription requires the general transcription factors that:
*assemble on the promoter
*position RNA polymerase
*pull apart DNA double helix to expose the template strand
*launch the RNA polymerase beginning of transcription
Transcription continues until the RNA polymerase reaches the termination (stop) signal
Stop (Termination) Signal:

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DNA base sequence that causes the RNA polymerase to detach from the DNA
Consists of a self-complimentary sequence followed by a region of poly-A bases
Processing of the RNA Transcript
In procaryotic cells, the primary RNA transcript is ready to function as messenger RNA
(mRNA) and be translated into a polypeptide
In eucaryotic cells, the primary RNA transcript requires further processing to become a
functional mRNA
Processing:
*takes place during transcription
*completed before RNA transcript id shipped to cytoplasm
*special enzymes carry processing reactions
*transcripts processed in various ways depending on RNA type
Processing steps on mRNA:
1. Capping of 5'-end
2. Polyadenylation
3. Splicing
1. Capping of 5'-end
*Addition of 7-methyl guanosine to the 5'-end of the transcript
*Cap attached by an unusual 5'-5' triphosphate bond
Capping occurs soon after the 5'-end (the front end) of the transcript emerges
from the RNA polymerase
2. Polyadenylation of the 3'-end
*An enzyme cuts the transcript at a specific sequence and removes part of the 3'-end of
the transcript after it is released from the RNA polymerase
*A second enzyme attaches multiple adenine nucleotides to the 3'-end of the transcript
(poly-A "tail")
These two modifications:
*Increase stability of mRNA molecule
*Aid mRNA export from the nucleus
*Identify RNA fragment as a fully completed mRNA
Splicing of the Primary Transcript
The majority of the primary transcript of RNA consists of noncoding sequences, which
are removed by splicing
Primary transcript typically 7000 - 20,000 nucleotides long
Only ~5% eventually translated into a polypeptide
95% consists of noncoding (intervening) sequences - introns
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