In the last lesson we discussed the
importance of DNA and the concept of a gene. Genes encode for all the
functional "stuff" of the cell but do not directly participate in the
work of cells' life-cycles. Think of it this way. If a cell can be
thought of as a car, the car is made up of various different parts that all
need to work in unison and at the right time, like the engine, brakes,
ignition, etc. Likewise, a car is defined by a set of instructions that
tell you have to make the car; in this case it is the car's blueprint. The
blueprint does not directly tell the car how to function but instead indirectly
serves as a guide for all the other components to tell them how to work and
come together. This is the case for DNA. DNA does not do any of the
“mechanical” work of the cell, but most commonly indirectly encodes for
proteins, which in turn serve as the “nuts and bolts” of the cell.
The conversion of a DNA gene sequence into a
protein occurs through an intermediate. This intermediate molecule is
called messenger ribonucleic acid or mRNA. mRNA like DNA is considered a
nucleic acid and is made up of nucleotides, though the nucleotides for DNA and
RNA are slightly chemically different. For instance, instead of thymine
in DNA, mRNA contains uracil in all places where thymine would have been found
in the DNA the mRNA originated from. There are many types of
RNA (and all are single-stranded unlike double-stranded DNA), but I am only
going to talk about mRNA.
As I have stated before, the genetic information of DNA lies in
the sequence of the four nucleotides: adenine, thymine, cytosine, and
guanosine. (I have chosen to provide a bit of a visual for this lesson below.)
This genetic information in the DNA is converted into mRNA in the process
called Transcription. mRNA serves as a complementary template according
to the nucleotide base-pairing rules of the gene on the DNA that it is encoding
from. For example if the gene sequence is ATCG, then the complementary
mRNA sequence would be TAGC. The entire gene is converted into its complementary
sequence into the mRNA intermediate. The mRNA is then used to tell the cell how
to make a given protein in the process of Translation, which I will be
discussing next.
Base Pairing Review:
A -> T
T -> A
G -> C
C -> G
*The letters on the first column represent the four nucleotides
that are found in DNA. The letters in the second column with the arrow
pointing towards them indicate the nucleotides that will base-pair with the
frist column. You can also see the Third Lesson for more review.
The Gene Example:
A --
T
T --
A
C --
G
G --
C
Notice that the gene in the DNA is double stranded!
The mRNA complement from the gene would be:
A -- T
- U
T -- A
--------> - A
C -- G
- G
G -- C
- C
Notice that the mRNA is single-stranded and complementary to the
gene sequence, and therefore matches up perfectly with one of the DNA strands!
Also notice that thymine is replaced by uracil!
Reference Videos:
Some Videos covering the Topic of Transcription:
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