Genetic code
The genetic code is the universal set of rules that explains how information stored in DNA is used to build proteins. This happens through a two-step process called the Central Dogma:
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1st step: DNA is transcribed into messenger RNA (mRNA). The mRNA is a working copy of the gene, and it’s transported out of the nucleus into the cytoplasm.
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2nd step: mRNA is translated into protein as ribosomes read the mRNA sequence.
Proteins are synthesized by ribosomes, and they represent the final expression of genetic information encoded in genes.
Translation relies on the triplet code: every three-nucleotide sequence (a codon) specifies either an amino acid or a signal to start or stop protein synthesis.
During translation, each codon on the mRNA pairs with a corresponding anticodon on transfer RNA (tRNA). This codon-anticodon relationship helps ensure the correct amino acid is added to the growing polypeptide chain.
The genetic code is described as degenerate, meaning most amino acids are encoded by more than one codon. This redundancy is partly explained by wobble pairing, where the third nucleotide of the codon can pair more flexibly with the corresponding nucleotide of the anticodon. As a result, some codon changes don’t change the amino acid that’s incorporated.
Translation begins at an initiation codon - typically AUG, which codes for methionine. As the ribosome moves along the mRNA, it reads codons in order and joins the corresponding amino acids to build a protein.
Errors in this process can result in different types of mutations:
- a missense mutation occurs when a codon is altered such that it codes for a different amino acid
- a nonsense mutation converts a codon into a stop signal, leading to premature termination of protein synthesis.
Translation ends when a termination codon (UAA, UAG, or UGA) is encountered, signaling the end of the polypeptide chain.
Messenger RNA (mRNA) structure and composition
- Messenger RNA (mRNA) is the link between DNA transcription and protein translation. It carries genetic instructions from the nucleus (or nucleoid in prokaryotes) to ribosomes. mRNA is made of RNA nucleotides arranged in an order that corresponds to the coding region of a gene.
Structural features of mRNA
Eukaryotic mRNA
In eukaryotic cells, mRNA undergoes post-transcriptional modifications that improve stability and support efficient translation. These modifications include:
- 5’ Cap: A specially modified guanine nucleotide is added to the 5’ end through an unusual linkage. The 5’ cap has several functions:
- Protects the mRNA from degradation by exonucleases
- Helps ribosomes recognize the mRNA and begin translation initiation
- Poly-A Tail: The 3’ end of mRNA is extended with a polyadenine (poly-A) tail, consisting of multiple adenine (A) residues. This structure:
- Shields the mRNA from degradation
- Helps with mRNA export from the nucleus and increases translation efficiency
The typical structure of a eukaryotic mRNA molecule can be summarized as:
5’ Cap - Coding Sequence - 3’ Poly-A Tail
Prokaryotic mRNA
Unlike eukaryotic mRNA, prokaryotic mRNA lacks both a 5’ cap and a poly-A tail. In bacteria, translation often begins while transcription is still happening. This is called coupled transcription and translation.
mRNA levels and gene expression
The amount of mRNA in a cell is directly related to protein synthesis levels: more mRNA generally leads to more protein production. Several factors influence mRNA levels:
- Promoter Strength: A strong promoter leads to higher transcription rates, producing more mRNA.
- DNA and Histone Methylation: In eukaryotes, DNA methylation and histone modifications help regulate transcription. Less methylation typically results in greater gene expression, leading to more mRNA production.