Download Genetics: The Science of Heredity

Genetics: The Science of
Heredity
Transcription
DNA vs.RNA: differences
• DNA (deoxyribose
nucleic acid) and
RNA (ribose nucleic
acid) are both
nucleotide polymers.
These molecules are
very similar but there
are some distinct
differences between
them.
• Both molecules are helical:
– DNA is a double helix
– RNA is a single helix.
• DNA bases:
– Adenine (A), Thymine (T),
Cytosine (C) & Guanine (G)
• RNA bases:
– A, G and C but T is replaced
with Uracil (U)
• DNA has one less oxygen on
the 5 carbon sugar than RNA;
thus the difference in their
names. Deoxyribose simply
refers to a ribose sugar lacking
an oxygen molecule.
DNA vs. RNA molecular difference
• The lack of one
oxygen molecule on
the DNA 5 carbon
sugar
RNA types
• 1. Ribosomal RNA (rRNA): make up
ribosomes
• 2. Transfer RNA (tRNA): transport
amino acids to ribosomes
• 3. Messenger RNA (mRNA): copied
from DNA, conveys information from
chromosomes to ribosomes
DNA vs. RNA: Similarities
• Both essential in protein
synthesis.
– Transcription: DNA is
transcribed into
Messenger RNA (mRNA).
– Translation: mRNA is
translated into a
polypeptide chain with the
aid of Ribosomal RNA
(rRNA) and Transfer RNA
(tRNA).
Transcription Essentials
• Transcription occurs in nucleus.
• Transcription: production of mRNA copy of the
DNA gene.
– Think of DNA as instructions to build hardware
(proteins), unfortunately, these instructions are in
another language and incomprehensible to the
workers that will eventually assemble the hardware.
This is where mRNA will come into the picture - to
provide new instructions that will be used by the
workers.
Steps of Transciption
1. Initiation: DNA is unzipped and the enzyme RNA polymerase runs along
the template strand of the DNA.
– The template strand of DNA can be identified by finding the promotor region:
nucleotide sequence T A C at the 3’ end (If the strand is written backwards it may
look like C A T at the 3’ end). This identifies that strand as the template and the
other strand, the information strand, will not be used in this transcription (this
does not mean, however, that it may not be used in future transcription
processes).
2. Elongation: As the RNA polymerase runs along the DNA template strand it
will add the complementary RNA nucleotides to the DNA nucleotides.
– This means that G will be paired with C, and visa versa, and A (DNA) will be
paired with U (RNA - rather than T in DNA replication) and T (DNA) paired with A
(RNA).
3. Termination: Transcription continues until RNA polymerase reaches a DNA
region called the termination signal: nucleotide sequence that marks the
end of a gene.
•
When the single helix mRNA strand is complete, RNA polymerase releases the
DNA andnew RNA molecule. The DNA will re-zip into the double helix.
The Process of Transcription
Diagrams of Transcription
Processing the Products of
Transcription
• In eukaryotes, once the mRNA is transcribed it
will then be processed.
– A cap and tail will be added to the ends of the mRNA
strand.
– The strand will be spliced.
• The introns (non-coding regions) will be removed
• The exons (coding regions) will be spliced together
– The completed mRNA strand has groups of three
nucleotides known as codons (for example, A U G is
the codon in mRNA that was transcribed from T A C).
These groups of three will code for a particular amino
acid in translation (A U G will code for the start amino
acid, methionine, in translation).