Answer to Question #191819 in Biochemistry for Darlington Chimwar

Question

Describe how genetic information is expressed as proteins. In you answer, describe DNA replication, transcription and translation. 

Answer 1

In living things, DNA is the reservoir of genetic information in tri-nucleotide sequences called codons that translates to specific amino acids. This genetic information is preserved in the DNA of new cells through replication in mitosis. Each strand of the double helix act as a template for the synthesis of the new complementary strand. During replication, the double helix DNA is unwound by topoisomerases and unzipped by helicase and a replication fork is formed. These give room for the attachment of RNA primer. DNA polymerase binds to the leading strand and walks through it adding nucleotide bases from the 5' to 3' direction. It also binds to the lagging strand leading to the formation of Okazaki fragments from 5’ to 3’ (Molecular mechanism of DNA replication (article) | Khan Academy, 2021). The nitrogenous bases are matched as follows, T with A and C with G. The primer is removed by exonuclease. Proofreading for errors is done by DNA polymerase. The nucleotide strands are joined by ligase in presence of ATP. After replication has stopped, the DNA winds up into a double helix.

In transcription, the genetic information from DNA is transferred to mRNA. This section of the DNA that is transcribed to mRNA is called a gene. The gene is used as a template. This template has a promoter, coding sequence, and terminator for identification by RNA polymerase complex. Transcription happens inside the nucleus in three steps that is initiation, elongation, and termination. In initiation, RNA polymerase binds to the promoter region causing the DNA to unwind. The RNA polymerase is now ready to read the bases in the gene. In elongation, RNA polymerase adds bases to the mRNA from 5' to 3' from the coding sequence. It replaces T in the gene With U in mRNA. In termination, RNA polymerase releases itself from the termination region after encountering a stop codon.

mRNA is processed through splicing, editing, the addition of 5' cap and 3'-polyadenylation tail. This enables the formation of more than one protein from a single gene (Foundation, 2021). Translation happens in the cytosol. Ribosomes are responsible for translating mRNA to the target protein, tRNA carries the right sequence of amino acids to the ribosome. Transfer RNA contains anticodon of the amino acid in transit. For example, amino acid serine has codon AGC therefore it will be carried by tRNA with anticodon UCG whenever AGC is translated in mRNA by the ribosome. Ribosomes consist of rRNA and proteins. They exhibit enzymatic activities. Ribosomal RNA has two subunits, a large subunit with E, P, and A active site and a small subunit. The translation is divided into three phases which include initiation, elongation, and termination.

In initiation phase the small subunit of ribosome bind and scan the mRNA from upstream to the downstream region until it encounters the start codon AUG. Large subunit attaches the tRNA with amino acid methionine on its P site.

In the elongation phase ribosome, move one codon at a time. Charged tRNA enters the ribosome complex and uncharged tRNA leaves. Therefore polypeptide chain grows one amino acid at a time. Energy from GTP is responsible for driving the elongation phase.

In the termination phase, termination of the polypeptide sequence occurs after the large subunit of the ribosome has encountered the stop codon UAG, UAA, and UGA. The growing polypeptide sequence is released, the mRNA is degraded and the nucleotides reused in other metabolic processes.

 

 

 

References

Khan Academy. 2021. Molecular mechanism of DNA replication (article) | Khan Academy. [online] Available at: <https://www.khanacademy.org/science/ap-biology/gene-expression-and-regulation/replication/a/molecular-mechanism-of-dna-replication> [Accessed 11 May 2021].

Foundation, C., 2021. CK12-Foundation. [online] CK-12 Foundation. Available at: <https://flexbooks.ck12.org/cbook/ck-12-college-human-biology-flexbook-2.0/section/5.6/primary/lesson/protein-synthesis-chumbio> [Accessed 11 May 2021].