Ed. 304-313; 7
Ed. 311-319; 368; 8
Ed. 325-336; 362-363.
You remember that RNA is similar to one strand of DNA, except that it has ribose instead of
deoxyribose as its sugar, and the base uracil (U) instead of T.
Importantly, DNA is always (except
in unusual cases such as in some viruses) double-stranded, and forms a double helix.
though RNA can base-pair, it does NOT form a double helix.
There are several different types of
RNA, with different functions.
The three most familiar are mRNA
(messenger RNA), (discussed
later in the lecture), tRNA
(transfer RNA), and rRNA
(ribosomal RNA) (both discussed next
However, cells also contain other types of RNA.
In many cases, RNA is present in large
complexes, together with protein molecules.
Ribosomes are an example.
Another example is
telomerase (discussed last time).
We’ll see another example (snRNPs and the spliceosome) later in
The functions of several types of RNA (including those in telomerase and the
spliceosome) have only been discovered in the last 20-30 years.
As an extreme example
, very short RNA molecules that regulate gene expression, were only discovered in
However, it is already clear that they are extremely important.
In fact, the 2006 Nobel Prize
in Medicine was awarded for the discovery of microRNA.
All RNAs are made by the same
mechanism, called transcription
, that we’ll discuss today.
Transcription is synthesis of RNA by inserting ribonucleotide triphosphates
(abbreviated NTPs) into a growing chain, by base-pairing them with a strand of DNA that's used as
(U in RNA base-pairs with A in DNA).
The enzyme that does this is called RNA
Bacteria have only 1 RNA polymerase, while eukaryotes have at least 3 different ones,
for making different kinds of RNA.
We’ll focus on the one that makes mRNA, called RNA
The mechanism of transcription is very similar to that of DNA synthesis.
However, there’s an
Each RNA molecule comes from transcription of ONE GENE.
not all genes are transcribed at once.
This is very different from DNA replication, where the entire
DNA molecule is copied.
Instead, we pick and choose which genes to transcribe at any time.
choose which genes to transcribe, we use a promoter
A promoter is a DNA sequence close to a
Each gene has its own promoter.
Binding of RNA polymerase and other proteins called
Transcription Factors to a promoter starts transcription of that gene.
These proteins bind to a
promoter only if the associated gene is to be transcribed.
We call the direction of transcription
“downstream”, and the opposite direction “upstream”.
That is, a promoter is upstream of its gene,
and RNA polymerase moves in a downstream direction along the DNA during transcription.
In bacteria, RNA polymerase binds directly to the promoter.