Chemical nature of genes & chromosomes (1 Viewer)

Eagle Mum

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@Hikeydropout, I’ve created this thread to post my response to this question which you asked in the thread that was accidentally deleted.

I looked up the HSC syllabus details for this topic which NESA provides in the public domain and can’t say that they are very helpful about what students need to know, so again my caveat that this is all relevant general information and you should ask your teachers about how to answer specific HSC questions.

The details you have written in your notes about the structure of DNA are correct but you should also consider the chemical interactions, specificity, function and activity of genes and chromosomes.

Chromosomes are complex structures comprising long double helix strands of DNA, consisting of four nitrogenous bases - two purines (adenine and guanine) and two pyrimidines (cytosine and thymine). Each nucleotide is composed of a nitrogenous base, a five-carbon sugar (deoxyribose), and a phosphate group. Within each strand, nucleotides bond covalently between the phosphate group of one and the deoxyribose sugar of the next. From this backbone extend the bases. Between the two strands of the double helix, the bases of one strand bond to the bases of the other with hydrogen bonds. Adenine always bonds with thymine, and cytosine always bonds with guanine. The bonding causes the two strands to spiral around each other in a double helix shape.

Within each chromosome, the linear DNA molecule wraps tightly as coils around proteins known as histones to form structures called nucleosomes which stack compactly onto each other to form fibers which further coil into a thicker and more compact structure. This unique structure of chromosomes and highly specialised chemical processes enable gene transcription without the DNA getting tangled up, whilst other proteins and enzymes maintain its supercoiled structure.

The functional units of chromosomes are genes. Each gene codes for a specific protein. Proteins (including structural proteins, transport proteins, enzymes, hormones etc), perform most of the essential functions in the body. This link (https://www.nature.com/scitable/topicpage/translation-dna-to-mrna-to-protein-393/) is to an explanation of transcription of DNA to mRNA and the translation of mRNA to amino acids which are the building of proteins. All of these processes are a series of very specific chemical reactions and reflect the ‘chemical nature’ of genes and their downstream products.

Genes comprise alternating segments of exons which are transcribed and introns which are not (or rather, they are spliced out from the pre-mRNA). At the beginning of each gene, there is a promoter region comprising a specific sequence of nucleotides, ‘TATA’, which is chemically recognised as the binding site of the TATA binding protein (TBP) and other transcription factors, to initiate gene transcription by the RNA polymerase II enzyme. The RNA polymerase enzyme together with the transcription factors bind to the DNA promoter region and generate a transcription bubble which locally separates the two strands of the DNA helix without disruption to the rest of the chromosome. This is done by breaking the hydrogen bonds between complementary DNA nucleotides within the transcription bubble. RNA polymerase then adds RNA nucleotides which are specifically complementary to the nucleotides of one DNA strand. After the RNA sugar-phosphate backbone forms with assistance from RNA polymerase to produce a stable RNA strand, the RNA–DNA bonds break, freeing the newly synthesized RNA strand.

Specific sequences of nucleotides are also chemically recognised as splice sites - ‘GT’ at the beginning or upstream part of an intron is called the donor splice site, whilst ‘AG’ at the end or downstream part of an intron is the acceptor splice site. The fact that these sequences are chemically recognised by the spliceosome, a unit of five small nuclear RNAs and several protein factors, means that mutations can create or destroy splice sites, forming abnormal (shorter or longer) mRNA molecules.

The chemical processes of demethylation and methylation/remethylation of DNA are employed by the cell to allow gene transcription when its protein product is required or to keep the gene quiescent when its protein products aren’t required.

Cell mitosis and meiosis, which I’ve already summarised elsewhere are also complex series of chemical processes.
 
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