A STORY OF NUCLEIC ACID BIOCHEMISTRY: TRANSCRIPTION

KEY POINTS:

Transcription is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule.
RNA polymerase is the main transcription enzyme.
Transcription begins when RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins).
RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule.
Transcription ends in a process called termination. Termination depends on sequences in the RNA, which signal that the transcript is finished.

INTRODUCTION

RNA polymerase is crucial because it carries out transcription, the process of copying DNA (deoxyribonucleic acid, the genetic material) into RNA (ribonucleic acid, a similar but more short-lived molecule).

Transcription is an essential step in using the information from genes in our DNA to make proteins. Proteins are the key molecules that give cells structure and keep them running. Blocking transcription with mushroom toxin causes liver failure and death, because no new RNA and thus, no new protein that can be made.

Transcription is essential to life, and understanding how it works is important to human health.

OVERVIEW OF TRANSCRIPTION

Transcription is the first step of gene expression. During this process, the DNA sequence of a gene is copied into RNA. Before transcription can take place, the DNA double helix must unwind near the gene that is getting transcribed. The region of opened-up DNA is called a transcription bubble.

Transcription uses one of the two exposed DNA strands as a template, this strand is called the template strand. The RNA product is complementary to the template strand and is almost identical to the other DNA strand, called the nontemplate or coding strand. However, there is one important difference in the newly made RNA, all of the T nucleotides are replaced with U nucleotides.

The site on the DNA from which the first RNA nucleotide is transcribed is called the plus, 1 site, or the initiation site. Nucleotides that come before the initiation site are given negative numbers and said to be upstream. Nucleotides that come after the initiation site are marked with positive numbers and said to be downstream.

If the gene that's transcribed encodes a protein which many genes do, the RNA molecule will be read to make a protein in a process called translation.

Transcription initiation

To begin transcribing a gene, RNA polymerase binds to the DNA of the gene at a region called the promoter. Basically, the promoter tells the polymerase where to "sit down" on the DNA and begin transcribing.

Each gene or in bacteria, each group of genes transcribed together has its own promoter. A promoter contains DNA sequences that let RNA polymerase or its helper proteins attach to the DNA. Once the transcription bubble has formed, the polymerase can start transcribing.

ELONGATION

Once RNA polymerase is in position at the promoter, the next step of transcription elongation can begin. Basically, elongation is the stage when the RNA strand gets longer, thanks to the addition of new nucleotides. During elongation, RNA polymerase "walks" along one strand of DNA, known as the template strand, in the 3' to 5' direction. For each nucleotide in the template, RNA polymerase adds a matching (complementary) RNA nucleotide to the 3' end of the RNA strand.

The RNA transcript is nearly identical to the non-template, or coding, strand of DNA. However, RNA strands have the base uracil (U) in place of thymine (T), as well as a slightly different sugar in the nucleotide. So, as we can see in the diagram above, each T of the coding strand is replaced with a U in the RNA transcript.

The picture below shows DNA being transcribed by many RNA polymerases at the same time, each with an RNA "tail" trailing behind it. The polymerases near the start of the gene have short RNA tails, which get longer and longer as the polymerase transcribes more of the gene.

TRANSCRIPTION TERMINATION

RNA polymerase will keep transcribing until it gets signals to stop. The process of ending transcription is called termination, and it happens once the polymerase transcribes a sequence of DNA known as a terminator.

DIFFERENCES BETWEEN REPLICATION AND TRANSCRIPTION