DNA's
role as the genetic material includes 1) carrying
information (in its base sequence), 2) copying that
information (replication), and 3) giving meaning to that
information (determining traits). DNA does this last job
by determining what proteins (including enzymes) are
made in the cell. Protein synthesis involves RNA
synthesis (transcription) and polypeptide synthesis
(translation). Crick proposed that his information
transfer was one way (The central dogma of genetics).
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- Genes and Polypeptides: Proteins are
made up of one or more polypeptides, which are
chains of amino acids joined by peptide
bonds. A gene can be defined as the DNA coding
for one polypeptide ("One gene, one polypeptide"). Yanofsky demonstrated
colinearity of the gene and polypeptide.
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- Transcription:
Transcription is DNA-directed RNA synthesis. The
sequence of a segment of a DNA molecule determines
the sequence of an RNA molecule. RNA is very similar
to DNA in structure, but is usually shorter, is
usually single stranded, has ribose in place of
2'-deoxyribose, and has uracil in place of thymine
(uracil base pairs with adenine just like thymine
does). Transcription is the process of making mRNA,
ribosomal RNA (rRNA), transfer RNA (tRNA), as well
as other small RNAs.
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- RNA Polymerase: The
enzyme that transcribes DNA is called RNA
polymerase. RNA polymerase uses the nucleoside
triphosphates (ATP, GTP, CTP, UTP) and RNA
polymerization occurs just like DNA polymerization
does. That is, it begins with the 5' end and the
new RNA molecule grows in the 5' to 3' direction
(a nucleotide is added to the 3'-OH). RNA
polymerase uses one strand of the DNA molecule as
the template strand with the 4 bases of DNA (A, G,
C, T) specifying which RNA nucleotides will be
added (U is added when the template DNA nitrogen
base is A). As with DNA replication, the
newly-made RNA molecule is antiparallel to the
template DNA. However, unlike DNA polymerase, RNA
polymerase does not need a primer but can add
start a new RNA molecule with a single NTP
(therefore, the first nucleotide of an RNA
molecule has 3 phosphates on its 5' end). Only one
strand of the DNA double helix is used as a
template. That template strand is used is called
the template strand and the strand not used is the
non-template strand. An RNA molecule made by
transcription is called a transcript.
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- Promoters and
Termination Sites: The site
where RNA polymerase binds and begins
transcription is called the promoter.
Transcription stops at a termination site.
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- RNA Processing: In
eukaryotes, pre-mRNA molecules undergo considerable
processing before leaving the nucleus and directing
translation. This includes the addition of a cap at
the 5' end, a poly-A tail, and splicing: the removal
of introns leaving in exons.
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Things I
Learned at the Movies:
Should you decide to defuse a bomb, don't worry which
wire to cut. You will always choose the right one.
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Lectures
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