Download Transcription Protein Synthesis So what does it mean? Transcription

10/16/2014
Protein Synthesis
Central Dogma of Biology:
Transcription
Date ________
Central Dogma
• Model that describes how DNA serves as a
genetic code for protein synthesis
• Proteins are the structural building blocks for
cells, and they act as enzymes
• Geneticists accept that the basic mechanism of
reading and expressing genes is DNA  RNA 
protein.
• This chain of events occurs in all living things, from
bacteria to humans.
• Scientists refer to this mechanism as the central
dogma of biology: DNA codes for RNA, which guides
the synthesis of proteins.
So what does it mean?
• When a protein is needed, the cell
must go through protein synthesis.
• DNA cannot leave the nucleus of
the cell, but proteins are made in
the cytoplasm
– How do we get proteins?!
• An intermediate must be
made – RNA is used to carry
the message from the nucleus
to the cytoplasm
RNA
Transcription
• RNA is a nucleic acid that is similar to DNA, however
RNA contains the sugar ribose, the base uracil replaces
thymine, and usually is single stranded.
• Three types:
• Purpose: make a copy of mRNA using DNA as a template. This
allows the recipe in DNA to make the protein in the
cytoplasm.
• mRNA base-pairs with DNA based on specific rules
– A—U, G—C
• The specific type of RNA formed during
this process is called mRNA (messenger)
• Only one gene is copied during transcription
• After transcription, the mRNA leaves the
nucleus and enters the cytoplasm; the two
strands of DNA join back together
– Messenger RNA (mRNA): long strands of RNA nucleotides
formed complementary to one strand of DNA. They travel
from the nucleus to the ribosome to direct synthesis of a
specific protein.
– Ribosomal RNA (rRNA): RNA that associates with proteins
to form ribosomes in the cytoplasm.
– Transfer RNA (tRNA): smaller segments of RNA nucleotides
that transport amino acids to the ribosome.
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How does it happen?
• Chemical signals trigger a gene for a specific protein
to turn ON
• RNA polymerase attaches to the exact location on
the DNA molecule where the gene is
• Two strands separate at that location
• RNA strand is made – this
is called mRNA,
specifically
4. Only a small part of the DNA double helix is
unwound/unzipped at a time – RNA polymerase
travels along the gene, bringing in RNA nucleotides
to base-pair with the existing DNA nucleotides along
the 3’  5’ leading strand, called the template
strand here.
- Uracil is incorporated instead of thymine,
because there is no thymine in RNA.
5. When RNA polymerase reaches the termination
signal at the end of the gene, it lets go of the DNA
and mRNA and leaves to transcribe elsewhere. The
mRNA travels out of the nucleus and into the
cytoplasm where it may be translated.
The Steps
1. A protein needs to be made from a gene
(DNA).
2. Proteins called transcription factors are
located at the beginning of the needed gene.
These tell RNA polymerase where to begin
transcribing.
3. As soon as RNA polymerase has positioned
itself at the beginning of the gene, it begins
transcription.
What happens once mRNA is made?
• It must go through processing before it can be used
to make a protein.
• The strand of mRNA is much shorter than the gene it
is copied from in DNA. Why?
– DNA sequences are made up of segments called introns
and segments called exons.
• Exons code for the gene. After transcription, the introns are
removed from the mRNA, leaving only the exons to code for the
protein.
• Other methods of processing include:
– 5’ cap at the beginning of mRNA
– Poly-A tail at the end of mRNA (sequence of repeating As)
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