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Coding and Non-Coding DNA

Biology (Year 12) - DNA Structure and Function

Ben Whitten

The Human Genome

Image: Wellcome genome bookcase image, Image by Russ London, Sourced Under a Creative Commons 4.0 License from Wiki Commons The entire order of the nucleotides found in a human cell's DNA has been sequenced thanks to the Human Genome Project, and this sequence of consecutive DNA nucleotide bases spanning all the chromosomes of a cell from start to finish is known as the genome. It's important to note that some sections of this genome code for proteins, and are called coding DNA, whereas all other DNA is non-coding.

Coding vs. Non-Coding DNA

The coding sections of DNA are also called genes, and they specify the sequences of amino acids in proteins (the building blocks of proteins). Proteins are responsible for nearly all cell functions! Humans have roughly 20,000 protein-coding genes, which is approximately 1-2% of all DNA in a human. Genes contain information for the production of proteins, which are the link between the stored genetic coding (the genotype) and the physical expression of traits (the phenotype). The majority of the human genome however is comprised of non-coding DNA. A genome is 'all the DNA in a cell', and includes the genes and the DNA that isn't part of any gene. The sections of DNA that don't code for any protein are classified as non-coding DNA, which can be further classified into functional non-coding RNA molecules (such as tRNA and regulatory RNA). Historically, non-coding DNA was simply referred to as 'junk DNA'! Scientists have now however found that some non-coding DNA is important, and isn't as 'junky' as they had originally thought.

The genetic code is simply the term used to describe how the four nitrogenous bases are ordered in DNA. The base order is read by cellular machinery and turned into a protein through protein synthesis. Cellular machinery consists of 'biological machines' that work hard to manufacture a biological molecule. In the genetic code, each set of three DNA nucleotides in a row counts as a triplet and codes for an mRNA (messenger RNA) triplet called a codon. The mRNA codon is read by cellular machinery and translated into a single amino acid, As we know, some proteins are made up of hundreds of amino acids; the code that would make one protein could have hundreds, sometimes even thousands of codons (triplets) contained within it.

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