Fourth Lesson: Transcription

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!

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