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<title data-react-helmet="true">Brad V Bellomo - A Primer on Molecular Biology</title> | ||
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<body> | ||
<h1>A Primer on Molecular Biology</h1> | ||
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<p><span class='term'>Genes</span> control the synthesis of <span class='term'>proteins</span>, which are the most important molecules making up all life. | ||
Life is not just protein, for example, an oak tree is made up mostly of polysaccharides. | ||
But it is genes and proteins that determine what makes an oak tree a tree and not a fish or a bird or a bacterium.</p> | ||
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<p>Genes are stored, replicated and passed on in <span class='term'>DNA</span>. DNA is <span class='term'>transcribed</span> into <span class='term'>messenger RNA</span> <span class="abbr">(mRNA)</span>. | ||
When a mRNA molecule reaches a <span class='term'>ribosome</span>, it is <span class='term'>translated</span> into <span class='term'>polypeptide chains</span>, which <span class='term'>fold</span> into proteins. | ||
DNA, RNA and polypeptide chains are all <span class='term'>linear polymers</span> – long molecules composed of smaller repeating subunits.</p> | ||
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<p><span class="abbr">RNA</span> is an abbreviation for <span class="term">RiboNucleic Acid</span>, as it is a long chain of repeating <span class="term">ribose</span> subunits, each of which has a <span class="term">nucleotide</span> attached. | ||
These nucleotides can be <span class="term">guanine</span> <span class="abbr">(G)</span> , <span class="term">uracil</span> <span class="abbr">(U)</span> , <span class="term">adenine</span> <span class="abbr">(A)</span> , or <span class="term">cytosine</span> <span class="abbr">(T)</span> . | ||
<span class="term">Hydrogen bonds</span> form nucleotides, most commonly 3 bounds between G and C and 2 bonds between U and T. | ||
These bonds can control the shape of the RNA molecule, forming different structures. | ||
The ribosome itself is mostly composed of RNA. | ||
Some scientists speculate that early life came from an <span class="term">RNA World</span>, as RNA could theoretically contain and copy genetic information as well as serve as the structure of organisms before DNA or proteins. | ||
Since ribose is not symmetrical, neither is RNA. Scientists refer to a <span class="term">3 Prime</span> <span class="abbr">(3’)</span> and <span class="term">5 prime</span> <span class="abbr">(5’)</span> end of RNA. | ||
These terms came from the convention of numbering Carbon atoms in a ribose molecule from 1’ to 5’. | ||
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<p><span class="abbr">DNA</span> is an abbreviation for <span class="term">DeoxyriboNucleic Acid</span>, and is similar to RNA. | ||
Deoxyribose is similar to ribose, but lacks an −OH, which in chemistry is called a <span class="term">hydroxyl group</span>, giving us the terms <span class="term">deoxy sugar</span> and <span class="term">Deoxyribose</span>. | ||
This difference in structure leads to DNA forming the well-known double-helix structure – a deoxyribose backbone with nucleotides is paired with another deoxyribose backbone with opposite nucleotides. | ||
This arrangement facilitates copying DNA or transcription, since when separating both strands, either strand contains all the genetic information of the pair, and either a new pair or messager RNA can be created. | ||
This structural change, however, limits the number of structures DNA can fold into, making an organism built entirely of DNA implausible. | ||
Similar to RNA, we can use the 3’ and 5’ convention to refer to direction, but can also use <span class="term">positive</span> and <span class="term">negative</span> strands, or <span class="term">coding</span> and <span class="term">template</span> strands. | ||
In DNA, uracil is replaced by <span class="term">thymine</span> <span class="abbr">(T)</span>. | ||
</p> | ||
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<p> | ||
During <span class="term">translation</span>, the RNA molecule structure is broken so the ribosome can read RNA as a sequence of nucleotides, this process is referred to as <span class="term">elongation</span>. | ||
The mRNA is read from the 3’ end. The first section of the mRNA typically doesn’t encode for a protein, and since it is towards the 5’ end, it is referred to as the <span class="term">5 prime untranslated region</span><span class="abbr">(5’ UTR)</span>. | ||
Nucleotides are translated in sequences of <span class="term">codons</span>, 3 nucleotides which identify a specific amino acid that should be added to a polypeptide chain to create the protein. The mRNA is read until the first <span class="term">start codon</span>, typically AUG. | ||
3 possible combinations of 4 possible nucleotides gives 64 possible combinations. | ||
With only 20 amino acids coded for in unexceptional circumstances, there is redundancy in this code, for example, ‘CUU, CUC, CUA and CUG all typically encode the amino acid Leucine. | ||
Each codon is associated with a <span class="term">transfer RNA</span><span class="abbr">(tRNA)</span>, which is a molecule of RNA specific to the codon that carries the amino acid. | ||
Normally each tRNAs must be available, if CUU is read and that tRNA is unavailable, it doesn’t matter if CUC CUA or CUG codons are available, translation will stop. | ||
A codon for a tRNA that doesn’t arrive in time typically aborts the translation and the partial protein product is recycled. | ||
Life has exceptions to rules, and there are some cases, known as wobble-pairs, where a different tRNA can be used that encodes the same animo acid. | ||
Nucleotides are read until a <span class="term">stop codon</span> (typically UAG, UAA, or UGA) is reached. Typically, there is still some RNA after the stop codon, and this region is referred to as the <span class="term">3 prime untranslated region</span><span class="abbr">(3’UTR)</span>. | ||
Typically, a single mRNA is read in parallel by several ribosomes at once. The translation process is remarkably similar between all known forms of life. | ||
</p> | ||
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