Molecular Biology’s Central Dogma

Learn.

Genetic information is stored in DNA and RNA – these are the universal to all living things. This universal language gives rise to all living things. This is the central dogma of molecular biology.

DNA can be transcripted into RNA. RNA can have a number of forms, however, messenger RNA (mRNA) is the type we will focus on the most. Other types include transfer RNA (tRNA), ribosomal RNA, microRNA and small nuclear RNA. Once the genetic information is transcripted into RNA it is then translated into a protein. This is done through linking amino acids. DNA can be copied over and over through the process of replication. Since most cells have their own copy of DNA, this is required for cellular reproduction. Reverse transcription can occur for RNA virus’ change their RNA into DNA so a cell can replicate them.

The genome, as we stated, is common to most cells. Exceptions being germline cells and mature red blood cells. The genome encompasses all the DNA in a cell.

The transcriptome encompases all the RNA in a cell. To cells may have different transcriptome, even the same cell at different times will have a different transcriptome. 

Proteome encompases all the proteins in a cell – like the transcriptome it can differ between cells and between different time points for the same cells.

Therefore, information can flow between the genome, transcriptome and proteome through transcription and translation. However, DNA is the most stable form of information storage since it has two copies so it knows how to correct a mistake as well. Cytosine deamination- when a cytosine is changed into a uracil – can be detected and corrected in DNA since uracil is not a base nucleotide for DNA.

Whereas, RNA is less stable since cytosine deamination is not corrected. However, this is ok for RNA since it is short lived and not reproduced the way DNA is – rather new RNA molecules are continually being remade.

Prokaryotic cells such as bacteria’s cells are smaller then eukaryotic cells. Additionally, they tend to have less genetic information (12 kb – 15 Mb). Where as, eukaryotic cells DNA is longer (10 Mb – 150 Gb) as well as being linear and arranged in chromosomes in a nucleus. As opposed to bacteria having free floating circular DNA.

Human cells are eukaryotic hence they contain linear DNA which is stored in chromosomes. Human’s have 22 pairs of chromosomes and 2 sex linked chromosomes. This is 6 billion base pairs long and encodes roughly 20,000 proteins.

Base pairs are read in triplets which do not overlap with one another. This results in 64 possible combinations of bases, however, only 20 amino acids are found naturally in biological systems. Hence this universal code of triplets can have different sequences for the same amino acids. This combination of three bases is known as a codon.

During mRNA transcription, DNA unravels in the region of the gene/ polypeptide being transcribed. This results in two strands of DNA being exposed. The template strand is the strand which the mRNA pairs up with – where there are A’s are paired with U’s, T’s are paired with A’s, C’s are paired with G’s, G’s are paired with C’s. This results in the mRNA sequence being the same as the non-template strand. Therefore, the codons are those for the non-template strand.

The Start codon is the AUG codon coding for the amino acid Met. UAG, UAA and UGA code for the stop codon. These set the reading frame for transcription initiating and stopping transcription. The mRNA that is transcribed is then translated into proteins following the same reading frame (reading from 5’to 3′) linking together amino acids to for a polypeptide/ protein.


Memorize.

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Master.

Question 1.

Explain how the central dogma of molecular biology accounts for information flow from DNA to proteins.

Question 2.

Does the genome, proteome and transcriptome differ between cells in the same human body. Explain why/why not for each point.

Question 3.

Appreciate the difference in size and construction between bacterial and eukaryotic genomes 

Question 4.

Define the universal genetic code and how this link to gene splicing in biotechnology

Question 5.

Explain, using reading frames, how a nucleic acid sequence can be transformed into a protein.

Answers.

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