Question

What is a gene? What does it mean for a gene to be expressed? What steps are necessary for gene expression to occur? How does gene expression result in you having a particular phenotype?

Answer 1

In terms of biology, a sequence of nucleotides of DNA or RNA that dictates the synthesis of a gene product, (either RNA or protein) is called a gene.

Gene expression is the process by which the information hidden in a gene is used in the synthesis of a functional gene product. It is like building a house from the blueprint. The gene products are generally proteins. But in non-protein-coding genes, for example, transfer RNA (tRNA) or small nuclear RNA (snRNA) genes, an RNA is the functional product, instead of a protein.

In a nutshell, the DNA is first copied into RNA by the process known as transcription. The RNA can be directly functional or be the intermediate template for protein production by translation. More detailed steps are as follows-

1. Transcription: The production of an RNA copy from a DNA strand. RNA polymerases, add one ribonucleotide after another to a growing RNA strand as per the complementarity law of the nucleotide bases. RNA is the messenger of cells that bring the information coded in DNA, from the nucleus to the cytoplasm (Eukaryotes).

2. mRNA processing: Transcription in prokaryotic protein-coding genes directly produce mRNA that is ready for translation into protein. But transcription of eukaryotic genes leaves a primary transcript of RNA (pre-RNA), which first has to undergo a series of modifications to become a mature RNA.

3. RNA export: In eukaryotes most mature RNA must be exported to the cytoplasm from the nucleus through the nuclear pores and into the cytosol.

4. Translation: Every mRNA consists of three parts: a 5' untranslated region (5'UTR), a protein-coding region or open reading frame (ORF), and a 3' untranslated region (3'UTR). The coding region carries information of protein synthesis in the form of genetic codes. Three subsequent nucleotides of the coding region forms a codon. Each corresponds to a binding site complementary to an anticodon triplet in transfer RNA. Transfer RNAs with the same anticodon sequence brings the corresponding amino acid. Amino acids are then chained together by the ribosome according to the order of triplets in the coding region. The ribosome stitch allthe amino acids brought according to the mRNA scquence in an unfolded polypeptide sequeence.

5. Folding: Each protein exists as an unfolded polypeptide or random coil when translated from a sequence of mRNA into a linear chain of amino acids. Thus it is non-functioal. The polypeptide then folds into its characteristic and functional three-dimensional conformation which is determined by the amino acid sequence.

6. Translocation: Secretory proteins of eukaryotes or prokaryotes must be translocated to enter the secretory pathway to send that to its the proper location of functioning.