Transcriptio

Promoters in Humans

In eukaryotes like humans, the main RNA polymerase in your cells does not attach directly to promoters like bacterial RNA polymerase. Instead, helper proteins called basal (general) transcription factors bind to the promoter first, helping the RNA polymerase in your cells get a foothold on the DNA. Many eukaryotic promoters have a sequence called a TATA box. The TATA box plays a role much like that of the -101010 element in bacteria. It's recognized by one of the general transcription factors, allowing other transcription factors and eventually RNA polymerase to bind. It also contains lots of As and Ts, which make it easy to pull the strands of DNA apart.

Transcription 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.

Template Strand

RNA polymerase uses one of the DNA strands (the template strand) as a template to make a new, complementary RNA molecule.

5' and 3'

The two ends of a strand of DNA or RNA strand are different from each other. That is, a DNA or RNA strand has directionality. At the 5' end of the chain, the phosphate group of the first nucleotide in the chain sticks out. The phosphate group is attached to the 5' carbon of the sugar ring, which is why this is called the 5' end. At the other end, called the 3' end, the hydroxyl of the last nucleotide added to the chain is exposed. The hydroxyl group is attached to the 3' carbon of the sugar ring, which is why this is called the 3' end. Many processes, such as DNA replication and transcription, can only take place in one particular direction relative the the directionality of a DNA or RNA strand.

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.

RNA polymerases

enzymes that transcribe DNA into RNA. Using a DNA template, RNA polymerase builds a new RNA molecule through base pairing. For instance, if there is a G in the DNA template, RNA polymerase will add a C to the new, growing RNA strand.

Transcription

the first step of gene expression. During this process, the DNA sequence of a gene is copied into RNA. is the process in which a gene's DNA sequence is copied (transcribed) to make an RNA molecule. an essential step in using the information from genes in our DNA to make proteins.

Transcription begins when

RNA polymerase binds to a promoter sequence near the beginning of a gene (directly or through helper proteins).