Gene Expression

The function of gene expression

Genes encode proteins, and proteins dictate cell function. Therefore, the combination of genes active in a particular cell determines the identity of a cell and its tasks. The activation of genes is called gene expression. It is tightly regulated at several points, from the initial stage of transcription to splicing and the translation of the proteins and the changes to the final protein structure. These processes are controlled to monitor and maintain cell types with required characteristics.

Not all parts of a gene code for the amino acid sequence of a protein molecule. The gene is composed of coding sections called exons and non-coding sections called introns. After the gene is transcribed into RNA, the introns are spliced out of the RNA molecule. Then the exons reattached to each other, forming a single mature mRNA molecule. This is splicing. The inclusion of introns or disruption of the order of exons in the mature mRNA molecule would produce a dysfunctional protein.

Therefore, gene expression uses information from genes to synthesise proteins or RNA molecules. The synthesis of proteins involves RNA in the processes of transcription and translation. You can read more about these vital processes in the Transcriptional Regulation and Translation Regulation explanations.

RNA in gene expression

Now, let's revise the structure and function of RNA, to understand its vital role in gene expression better.

RNA differs from DNA in the following ways:

Table 1. The main differences between RNA and DNA.

Three forms of RNA are involved in the translation and transcription of genes in a cell. These three forms are:

• Messenger RNA (mRNA) carries a copy of the gene to be expressed from the nucleus to the ribosome, where it can be translated.
• Transfer RNA (tRNA) consists of only one strand of RNA, which folds because of base pairing. Each tRNA molecule carries its specific amino acid to the ribosome.
• Ribosomal RNA (rRNA), rRNA, together with proteins, forms ribosomes.

Genes

DNA in genes contains instructions, consisting of non-coding instructions (sequences of DNA) and code for RNA or proteins. Coding regions of a gene are called exons, and non-coding ones are called introns. The DNA code consists of nitrogenous bases: adenine, cytosine, guanine, and thymine. Different base pair combinations create different amino acids and therefore produce various proteins.

A codon (three bases) codes for one amino acid. There are 20 amino acids and four bases (64 codons possible). Amino acids are specified by more than one codon, referred to as redundancy. Not all mutations will change the amino acid sequence of proteins; some mutations might still code for the same amino acid.

Cells that can express every one of their genes are stem cells. Stem cells can become any cell in the body because they can produce all the proteins needed to create a specific structure and carry out particular tasks. You can find more detail on this subject in the stem cells article.

The development of stem cells into particular body cells can occur due to the factors involved in transcription and translation. This regulation of protein synthesis means that different genes are expressed, and various proteins are made, enabling the development of stem cells into different body cells. Epigenetic control of genes determines whether a gene is expressed via altering how proteins and enzymes involved in transcription can transcribe the DNA. Uncontrolled cell growth can create mutations, which may develop into tumours. Tumours can be harmful and non-harmful, but you'll find more information about tumours in the article on Epigenetics.

Protein Synthesis/Stage 1 of Gene Expression

DNA helicase unwinds the DNA double helix, and the hydrogen bonds between base pairs are broken. RNA polymerase creates a complementary template strand. The result of this process is a molecule called pre-mRNA.

This pre-mRNA molecule is 'spliced', where introns are removed, and the remaining exons are re-joined. The molecule is referred to as mRNA after spicing.

1. During translation, the mRNA leaves the nucleus. The template strand travels to the ribosome to which it attaches.
2. Transfer RNA (tRNA) brings free amino acids from the cytoplasm to the mRNA in the ribosome. The codon on mRNA forms complementary base pairs with the anticodon of the tRNA molecule allowing the tRNA to specify the amino acid. The specific amino acid is bound to the other end of the tRNA molecules.
3. tRNA molecules are attached to the mRNA via the ribosome, which moves along the mRNA strand. As the ribosome covers two codons, two tRNA molecules are attached, and a peptide bond forms between them.
4. The amino acids form a polypeptide chain, and as each amino acid is attached, they separate from the tRNA. This allows the tRNA molecule to return to the cytoplasm and bring another amino acid.
5. The protein (polypeptide chain) formed can be changed by combining polypeptide chains or adding a phosphate or carbohydrate group.

Every protein has a unique shape designed to carry out a specific function. The primary structure, i.e. the sequence of amino acids, allows every protein to have a unique shape as there is nearly an unlimited number of possible amino acid combinations. The secondary structure of proteins can take one of two shapes: the alpha-helix or beta-pleated sheets.

If you want to learn about the tertiary structure of proteins, you can find it in the article on protein structure.

The quaternary structure of proteins consists of multiple polypeptide chains. The quaternary structure might contain prosthetic groups. These are non-protein groups that become integrated into the chains. This forms what is called a conjugated protein, e.g., haemoglobin. The primary structure dictates the secondary, tertiary and quaternary structures.

The disturbance of the primary structure completely changes the protein's function, highlighting the importance of the amino acid sequence.

Fig. 1 - Protein structure

Regulation of gene expression

Many of the genes in humans code for factors that regulate the transcription and translation of different genes. These factors control which genes are expressed and used to make proteins, and control which genes are not. The sequences that do not create a protein have the essential function of regulating the expression of other genes.

One such factor involved in gene regulation is an activator. Proteins must bind to the promoter region at the start of the DNA to be transcribed. Activator proteins such as RNA polymerase bind to the promoter legion and activate transcription.

Regulatory elements can have general or specific effects. These effects are described in the article on regulating transcription and translation. The regulation of translation involves RNA interference, which causes mRNA not to be translated into proteins. RNA interference uses siRNA (small interfering RNA in mammals) and miRNA (micro interfering RNA in mammals and plants). Refer to the article on regulation of translation for more detail.