Transcription and the Genetic Code From DNA to mRNA

The ‘Central Dogma’ The fundamental idea of molecular biology is: DNA  mRNA  Protein Q: Where does transcription fit in? A: Transcription is the synthesis of mRNA from a DNA template.

One gene = one ? It used to be generally believed that ‘one gene = one protein.’ This is now known to be untrue. Q: What makes this untrue and what might we say instead? A: Sometimes a protein is composed of multiple polypeptide strands (like hemoglobin) and thus we might say ‘one gene = one polypeptide.’ We also now know that some genes code for RNA products which themselves can do cellular work. Thus we might say that ‘one gene = one RNA product and usually a polypeptide’

RNA (Ribonucleic Acid) vs. DNA Q: How is RNA different from DNA? A: RNA contains the nitrogenous base Uracil (U) instead of Thymine (T). RNA is composed of ribose instead of deoxyribose RNA is generally single stranded

mRNA mRNA (messenger RNA) is the molecule that carries the instructions located on the DNA within the nucleus to the protein-making machinery in the cytosol of the cell.

Making RNA Like replication, there is one key enzyme responsible for synthesizing RNA and a bunch of supporting molecules that make synthesis possible. The star of the show is RNA polymerase

RNA polymerase RNA polymerase is an enzyme complex that runs along DNA, reading one strand (the template strand) and synthesizes a complementary strand of RNA. RNA polymerase, like DNA polymerase, reads 3’ to 5’ and synthesizes 5’ to 3’. In Eukaryotes there are 3 types of nuclear RNA polymerase and additional RNA polymerases in mitochondria and chloroplasts.

Binding: The Promoter The ‘beginning’ of a gene contains specific sequences called ‘promoter sequences.’ These sequences serve as a signal for RNA polymerase and transcription factors to bind. Most common promoter sequence is the ‘TATA Box.’ It is located at the -10 position. (count up 10 spots for the start)

Elongation: RNA Polymerase Once all the proper transcription factors are in place, RNA polymerase is ‘released’ and proceeds down the DNA, synthesizing the mRNA transcript. It does this at about 60 nucleotides/second

Termination: A Hairpin Loop RNA polymerase will stop transcription when it reaches a particular termination sequence. In some cases, this sequence (usually a GC rich region) codes for a segment of RNA that base pairs with itself in a hairpin loop fashion. This hairpin structure interacts with the RNA polymerase, causing it to fall off the DNA.

The mRNA transcript The mRNA transcript serves as the carrier of the DNA code. It is not yet ‘mature,’ and must be altered before it can direct protein synthesis. We will talk about this later…

The CODE Q: What is the ‘code’ that DNA and RNA are using to transmit their information? A: It is a ‘triplet’ code. Each sequence of three bases corresponds to an amino acid. Each of these sequences is called a codon.

The Triplet Code With 64 possible combinations (4x4x4), each of the 20 amino acids has a number of different codons that correspond to it. Explain how this fact relates to the genetic code being referred to as redundant, but NOT ambiguous.

Universality! Adding to our theory that all life is related through common ancestry, the genetic code happens to be nearly universal. Not all organisms use all the codons, but all organisms use the same sequences for the codons that they do use.

The Amino Acids Remember these? Proteins are made of them… There are 20 ‘essential’ amino acids – essential meaning that they are needed for life. (there are some other derivatives of the 20 amino acids that are unique to certain species)

Tryptophan and Turkey Q: Does tryptophan really make humans sleepy? A: Yes, but not during Thanksgiving…

Serotonin Serotonin is a hormone in our brains that has a calming, or ‘sleepy’ effect. Serotonin is synthesized from tryptophan. It has been found that high doses of tryptophan on an empty stomach can induce sleepiness. However, the effects of tryptophan on a full stomach with a ton of other amino acids and a lot of other food seem to be negligible.