Download UNIT 8 NOTES – MOLECULAR BIOLOGY AND EMBRYONIC

UNIT 8 NOTES – MOLECULAR BIOLOGY AND EMBRYONIC DEVELOPMENT
FROM GENES TO PROTEINS
I.
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Overview
Although all cells with a nucleus in our body contains the same genetic information but the
same set of genes may be expressed very differently at different stages in an organism’s life or
in different cells.
Mendel discovered that traits are passed from one generation to the next or could skip a
generation.
Hershey and Chase discovered that DNA is responsible for passing the genetic information on
from one generation to the next.
Archibald Garrod – as he studied alkaptonuria, he observed that some diseases are caused by
defective enzymes that stopped metabolic pathways.
Beadle and Tatum – used bread mold to cause mutations in them. Came up with the “One gene
– one enzyme hypothesis” with a set of experiments followed the procedure below:
As more information was discovered, this hypothesis was modified to one gene only determines
one polypeptide, not necessarily a fully functioning protein.
II.
The Central Dogma of Biology
 Francis Crick created the term. According to the central dogma information is always inherited
from the DNA to the RNA to the proteins.
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DNA is considered a permanent molecule in the cell’s life while RNA is temporary and breaks
down quickly after it is used.
Different types of cells make different proteins depending on their functions. So different genes
are active in them.
Review the structure of RNA and its types (mRNA, tRNA, rRNA and siRNA)
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III.
Transcription is the process that converts the information from the DNA into RNA.
Translation converts the information from RNA to proteins.
Transcription
A. Molecular Components:
 An enzyme called RNA polymerase opens the two strands of the DNA molecule and hooks
together the RNA nucleotides as they base-pair along the DNA. RNA polymerase can only
assemble the polynucleotide chain from the 5’ → 3’ direction but they don’t need priming to
start the assembling. ONLY THE 3’
5’ DNA TEMPLATE IS COPIED.
 There are specific regions on the DNA where the assembling of the new mRNA molecule starts.
The sequence where RNA polymerase attaches and initiates transcription is the promoter. In
prokaryotes, the sequence that ends transcription is called the terminator. The promoter
region is said to be “upstream” from the terminator region. The stretch of DNA that is being
transcribed into an mRNA molecule is called the transcription unit.
B. Synthesis of an RNA Transcript:
 The three stages of transcription are: initiation, elongation and termination of the RNA chain.
 Initiation: It starts at the promoter region of a gene. This region includes the actual start point
of transcription and several dozen other nucleotides “upstream”. The promoter region binds
the RNA polymerase and determines which DNA strand will be copied. In prokaryotes the RNA
polymerase directly recognizes the promoter region and binds to it. In eukaryotes, a collection
of proteins called transcription factors mediate the binding and initiates transcription. The
transcription factors, RNA polymerase and the promoter region together are called
transcription initiation complex. The specific region of the promoter that has a set of repeating
TATA bases is called the TATA box.
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Elongation: RNA polymerase moves along the DNA molecule, it continues to untwist the DNA 10
– 20 nucleotides at a time and adds nucleotides to the 3’ end of the RNA molecule. The
nucleotides are used in a form of ATP, GTP, UTP and CTP, so energy is provided by them to form
the new bonds of the forming RNA molecule. Several polymerase molecules can transcribe the
same gene at the same time. Once the RNA molecule is ready, it peels off of the DNA molecule
and the DNA twists back again.
Termination: This mechanism is different in prokaryotes and eukaryotes. In prokaryotes, the
process of transcription continues through the terminator sequence of DNA. This sequence
makes the RNA polymerase detach from the DNA molecule and release the transcript which is
completely done. In eukaryotes, the pre-mRNA is made by the RNA polymerase but there is a
long additional sequence of polyadenilation signal (AAUAAA) and other additional nucleotides
“downstream” from the original gene that was copied. These additional nucleotides make the
proteins that were associated with the transcription fall off and the pre-mRNA released.
However, the pre-mRNA has to go through an editing process before it can be used as a
functional mRNA molecule.
IV.
RNA modification
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Alteration of mRNA ends: The 5’ end that is transcribed first gets a modified guanine nucleotide
forming a 5’ cap. The 3’ end gets additional 50 – 250 adenine nucleotides forming a poly-A tail.
The 5’ cap and the poly-A tail seem to facilitate the export of the mature mRNA from the
nucleus. They also help to protect the mRNA molecule from degradation by hydrolytic enzymes.
They also help ribosomes to attach to the 5’ end of the mRNA molecule.
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RNA splicing: the removal of a large portion of the RNA molecule by a “cut-and-paste” method.
The removed noncoding sequences of mRNA nucleotides that lie between the coding regions
are called introns. Exons are the coding sequences. The signal for cutting is a set of small
sequences of nucleotides that are recognized by small nuclear ribonucleoproteins (snRNP’s or
“snurps”). These snurps join with other proteins to form a spliceosome – molecular units that
cut introns out and attach exons to each other.
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The consequence of DNA splicing is that a single gene can encode more than one kind of
polypeptide. Depending on which segment of the gene is treated as an exon, it can give rise to
multiple polypeptide – alternative RNA splicing.
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Great, detailed movie on transcription: http://vcell.ndsu.nodak.edu/animations/transcription/movie.htm
Same site on RNA processing: http://vcell.ndsu.nodak.edu/animations/mrnaprocessing/movie.htm
Great detailed animation with explanation on the entire process:
http://207.207.4.198/pub/flash/26/transmenu_s.swf
RNA splicing: http://www.sumanasinc.com/webcontent/animations/content/mRNAsplicing.html
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