Transcription and Translation (Video)

0:11 From gene to protein

First, we will go from gene to protein. A cell stores its genetic information as a DNA sequence, and proteins carry out most of the work in the cell. Transcription and translation are the two steps that turn the DNA sequence of a gene into a protein. Francis Crick set out the central dogma in 1958: sequence information passes from DNA to RNA to protein, and not back from protein. Transcription copies a gene into messenger RNA, or mRNA. Translation reads that mRNA and joins amino acids into a polypeptide chain. In bacteria, both steps happen in the cytoplasm, and a ribosome can start translating an mRNA while transcription is still running. In eukaryotes, transcription happens in the nucleus and translation in the cytoplasm.

1:13 RNA polymerase and the promoter

Second, we will look at RNA polymerase and the promoter. RNA differs from DNA in three ways that matter here. Its sugar is ribose, it uses uracil in place of thymine, and it is usually single-stranded. Transcription starts at a promoter, a DNA sequence just upstream of the gene. In bacteria, a sigma factor helps RNA polymerase recognize the promoter. In eukaryotes, the general transcription factor TFIID binds the TATA box, where a promoter has one, and RNA polymerase II joins after the other factors. RNA polymerase then unwinds about 12 to 14 base pairs of DNA and opens a transcription bubble. The enzyme reads the template strand from 3' to 5' and builds RNA from 5' to 3'. RNA polymerase can start a new chain on its own, so transcription needs no primer. The new RNA matches the coding strand, the non-template strand, with U in place of T.

2:14 Elongation, termination and processing

Third, we will follow elongation, termination and processing. Once the bubble is open, RNA polymerase moves along the gene and adds about 40 to 80 nucleotides per second in bacteria. In bacteria, transcription stops at a terminator. In the common intrinsic type, the new RNA folds into a GC-rich hairpin followed by a run of uracil, and the polymerase releases the RNA. A eukaryotic transcript needs further work before translation, so the cell processes this primary transcript, or pre-mRNA, before it leaves the nucleus. The cell adds a cap of 7-methylguanosine at the 5' end. It adds a poly-A tail of about 200 adenines at the 3' end. The spliceosome then removes the introns and joins the exons.

3:04 The genetic code

The fourth part covers the genetic code. The finished mRNA carries its information in codons, groups of three nucleotides, and translation reads the codons in order. Four bases taken three at a time give 64 codons. Of these, 61 specify the 20 amino acids, and three are stop codons, UAA, UAG and UGA. The codon AUG codes for methionine and is the usual start codon. The code is redundant, so most amino acids have more than one codon. For example, glycine has four codons, GGU, GGC, GGA and GGG. The codon assignments come from experiments in the 1960s. In 1961, Marshall Nirenberg and Heinrich Matthaei added a synthetic RNA that contained only uracil to a cell-free extract of Escherichia coli, and the extract made a polypeptide that contained only phenylalanine. So the codon UUU codes for phenylalanine.

4:09 Transfer RNA and the ribosome

Fifth, we will look at transfer RNA and the ribosome. To read the codons, the cell uses transfer RNA, or tRNA, as an adapter between codon and amino acid. Each tRNA has an anticodon that pairs with a codon, and an aminoacyl-tRNA synthetase attaches the matching amino acid to the tRNA's 3' end. The ribosome contains ribosomal RNA and proteins, in a small subunit and a large subunit. Bacteria use 30S and 50S. Eukaryotes use 40S and 60S. The small subunit binds the mRNA and the initiator tRNA carrying methionine at the start codon, and then the large subunit joins. The ribosome has three tRNA sites: A for aminoacyl, P for peptidyl and E for exit. A new tRNA enters the A site. The ribosomal RNA of the large subunit forms a peptide bond between the growing chain and the new amino acid. The ribosome then moves one codon along the mRNA, the empty tRNA shifts from the P site to the E site and leaves, and the growing chain sits in the P site. A bacterial ribosome adds about 15 to 20 amino acids per second.

5:29 Termination and a worked example

The last part covers termination and a worked example. Elongation continues until the ribosome meets a stop codon. When a stop codon reaches the A site, a release factor binds, the ribosome releases the polypeptide, and the subunits separate. For a worked example, take the coding strand ATG GCT TGG TAA. The mRNA has the same sequence, with U in place of T. If you read the mRNA in codons, AUG is methionine, GCU is alanine, UGG is tryptophan, and UAA is a stop codon. The polypeptide is methionine, alanine, tryptophan. Here is a summary of what you learned. RNA polymerase transcribes the template strand of a gene into mRNA and needs no primer. In eukaryotes, the cell adds a cap and a poly-A tail and removes the introns before the mRNA leaves the nucleus. The ribosome then reads the mRNA one codon at a time, and transfer RNAs bring the matching amino acids until a stop codon reaches the A site. Now try a question on what you learned. If a coding strand reads ATG TTT TAG, what polypeptide does the ribosome make from its mRNA? Pause here and try it before the answer appears. The mRNA reads AUG UUU UAG, so the ribosome makes methionine followed by phenylalanine, and UAG stops translation.