Central Dogma of Molecular Biology

Protein synthesis requires two steps: transcription and translation.

DNA contains codes

Three bases in DNA code for one amino acid. The DNA code is copied to produce mRNA. The order of amino acids in the polypeptide is determined by the sequence of 3-letter codes in mRNA.

DNA vs RNA

DNA
RNA

Sugar:
deoxyribose
ribose

Bonds with Adenine:
thymine
uracil

# of Strands:
two
one

Kinds of RNA

Messenger RNA (mRNA)
Messenger RNA contains genetic information. It is a copy of a portion of the DNA.
It carries genetic information from the gene (DNA) out of the nucleus, into the cytoplasm of the cell where it is translated to produce protein.
Ribosomal RNA (rRNA)
This type of RNA is a structural component of the ribosomes. It does not contain a genetic message.
Transfer RNA (tRNA)
Transfer RNA functions to transport amino acids to the ribosomes during protein synthesis.

Transcription

Transcription is the synthesis of mRNA from a DNA template.

It is like DNA replication in that a DNA strand is used to synthesize a strand of mRNA.
Only one strand of DNA is copied.
A single gene may be transcribed thousands of times.
After transcription, the DNA strands rejoin.

Steps involved in transcription

RNA polymerase recognizes a specific base sequence in the DNA called a promoter and binds to it. The promoter identifies the start of a gene, which strand is to be copied, and the direction that it is to be copied.
RNA polymerase unwinds the DNA.
RNA polymerase assembles bases that are complimentary to the DNA strand being copied. RNA contains uracil instead of thymine.
A termination code in the DNA indicates where transcription will stop.
The mRNA produced is called a mRNA transcript.

Processing the mRNA Transcript

In eukaryotic cells, the newly-formed mRNA transcript (also called heterogenous nuclear RNA or hnRNA) must be further modified before it can be used.
A cap is added to the 5’ end and a poly-A tail (150 to 200 Adenines) is added to the 3’end of the molecule.
Eukaryotic genes contain regions that are not translated into proteins. These regions of DNA are called introns and must be removed from mRNA. Their function is not well understood.
The remaining portions of DNA that are translated into protein are called exons. After intron-derived regions are removed from mRNA, the remaining fragments- derived from exons- are spliced together to form a mature mRNA transcript.

The Nucleus

DNA is located in an organelle called the nucleus.
Transcription and mRNA processing occur in the nucleus.
The nucleus is surrounded by a double membrane. After the mature mRNA transcript is produced, it moves out of the nucleus and into the cytoplasm through pores in the nuclear membrane.

Translation

Translation is the process where ribosomes synthesize proteins using the mature mRNA transcript produced during transcription.

Overview

The diagram below shows a ribosome attach to mRNA, and then move along the mRNA adding amino acids to the growing polypeptide chain.

Translation - Details

A mature mRNA transcript, a ribosome, several tRNA molecules and amino acids are shown. There is a specific tRNA for each of the 20 different amino acids.
Below: A ribosome attaches to the mRNA transcript.
A tRNA molecule transports an amino acid to the ribosome. Notice that the 3-letter anticodon on the tRNA molecule matches the 3-letter code (called a codon) in the mRNA. The tRNA with the anticodon "UAC" bonds with methionine. It always transports methionine. Transfer RNA molecules with different anticodons transport other amino acids.

A second tRNA molecule bonds to the mRNA at the ribosome. Again, the codes must match.

A bond is formed between the two amino acids.
The tRNA bonded to methionine drops off and can be reused later.
The ribosome moves along the mRNA to expose another codon (GAU) for a tRNA molecule.
The only tRNA molecule that can bond to the GAU site is a molecule with a CUA anticodon. Transfer RNA molecules with CUA anticodons are specific for asparagine.
Asparagine is now added to the growing amino acid chain.