Chapter 28 – Infographic descriptions

28.4a What makes up the Chemical Structure of DNA?

The sugar-phosphate backbone. DNA is a polymer made up of units called nucleotides. The nucleotides are made of three different components: a sugar group, a phosphate group, and a base. There are four different bases: adenine thymine and cytosine. DNA strands are held together by hydrogen bonds between bases of adjacent strands. Adenine (A) always pairs with thymine (T), while guanine (G) always pairs with cytosine (C). Adenine pairs with uracil (U) in RNA.

[latex]\text {DNA}\xrightarrow[Transcription]{}\text{RNA}\xrightarrow[Transcription]{}\text{Protein}[/latex]

The bases on a single strand of DNA act as a code. The letters form three letter codons, which code for amino acids – the building blocks of proteins.

An enzyme, RNA polymerase, transcribes DNA into mRNA (messenger ribonucleic acid). It splits apart the two strands that form the double helix, then reads a strand and copies the sequence of nucleotides. The only difference between the RNA and the original DNA is that in the place of thymine (T), another base with a similar structure is used: uracil (U).

• DNA sequence: T-T-C-C-T-G-A-A-C-C-C-G-T-T-A
• mRNA sequence: U-U-C-C-U-G-A-A-C-C-C-G-U-U-A
  • Phenylalanine: U-U-C
  • Leucine: C-U-G
  • Asparagine: A-A-C
  • Proline: C-C-G
  • Leucine: U-U-A

In multicellular organisms, the mRNA carries genetic code out of the cell nucleus, to the cytoplasm. Here, protein synthesis takes place. ‘Translation’ is the process of turning the mRNA’s ‘code’ into proteins. Molecules called ribosomes carry out this process, building up proteins from the amino acids coded for.

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28.4b Today in Chemistry History – Rosalind Franklin and the structure of DNA

Franklin was born 25 July 1920 and died 16 April 1958.

Rosalind Franklin was a chemist and X-ray crystallographer whose work was instrumental in the discovery of the structure of DNA. She missed out on a Nobel Prize for her work as they are not awarded posthumously.

DNA is a polymer made up from monomers called nucleotides. A sugar phosphate backbone forms the two strands, which are held together by hydrogen bonds between the bases found on these strands. Photograph 51 is an X-ray diffraction image of DNA taken during Franklin’s research. It was crucial in developing a model of DNA and confirming its double helical structure.

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28.4c The 2020 Nobel Prize in Chemistry: Using genetic scissors to edit the genome

The 2020 Noble Prize in Chemistry was awarded to Emmanuelle Charpentier and Jennifer A. Doudna for the development if CRISPR-Cas9 genetic scissors, a method for genome editing.

CRISPR stands for clustered regularly interspaced short palindromic repeats. It refers to repeated sequences in bacteria and archaea DNA. The sequences are part of an immune system; if a bacterium survives a viral infection, it adds a section of the virus genetic code to the CRISPR region of its own to serve as a memory in case it’s infected again. Charpentier and Doudna saw this could be used for gene editing.

CRISPR gene editing process is as follows:

1. Create a strand of guide RNA matching the DNA sequence where we want to make a cut and use a scissor protein, Cas9, to bind to the guide RNA.
2. The guide RNA searches for the target section of the DNA, transports the scissor protein to it, and the scissor protein cuts the DNA at this point.
3. The cell will try and repair the cut DNA, but this process is error-prone – disrupting the gene function. If we add a template, the cell will use this to carry out the repair, allowing us to edit the genetic code.

The ability to edit genomes has been used in plant breeding. Clinical tries underway to use this in therapies to treat some cancers and hopefully will lead to cures for some inherited diseases.

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28.5a The Chemical Structures of Vitamins

Vitamins are essential nutrients that our body needs in small amounts. An organic compound is defined as a vitamin when it is required by an organism, but not synthesized by that organism in the required amounts (or at all). There are 13 recognized vitamins, these can be divided broadly into two classes: water-soluble vitamins and fat-soluble vitamins.

Water-soluble vitamin are not stored in the body and are generally required more frequently than the fat-soluble vitamins.

Fat-soluble vitamins are stored in the liver and fatty tissues until required, therefore they can be harmful if too much is taken in.

Listed below are the water-soluble vitamins:

1. Vitamin B1 (Thiamin): Can also occur in pyrophosphate ester form. Used to keep nerve and muscle tissue healthy, and important for processing of carbohydrates and some proteins.
2. Vitamin B2 (Riboflavin): Excess turns urine bright yellow. Important for body growth, red blood cell production, keeping the eyes healthy, and helps processing of carbohydrates.
3. Vitamin B3 (Nicotinic acid and Nicotineamide): Niacin is collective name for these compounds. Helps with digestion, digestive systems health and helps with the processing of carbohydrates.
4. Vitamin B5 (Pantothenic acid): Also occurs in pyrophosphate ester form. Important for manufacturing red blood cells, maintaining a healthy digestive system, helps process carbohydrates.
5. Vitamin B6 (Pyridoxal phosphate): Active form in mammalian tissue. Helps make some brain chemicals needed for normal brain function and also helps make red blood cells and immune system cells.
6. Vitamin B7 (Biotin): Produced by intestinal bacteria. Needed for metabolism of various compounds, often recommended for strengthening hair though evidence varies.
7. Vitamin B9 (Folic acid): Found as tetrahydrofolate in food. Important for brain function and mental health, aids production of DNA and RNA, important when tissues are growing quickly.
8. Vitamin B12 (Cobalamin): Usually contains CN as the R group. Important for the nervous system, for making red blood cells, and helps in the production of DNA and RNA.
9. Vitamin C (Ascorbic acid): Deficiency causes scurvy. Important for a healthy immune system, helps produce collagen (used to make skin and other tissues), and helps wound healing.

Listed below are the fat-soluble vitamins:

1. Vitamin A (Retinol): Is an active form in mammalian tissues. It is important for eyesight, strengthens the immune system, keeps skin and lining pf [arts of the body healthy.
2. Vitamin D (Cholecalciferol): There is a difference between natural form and form used in supplements. Important for bone health and maintaining the immune system function, may also have a preventative role in cancers.
3. Vitamin E (Alpha-tocopherol): Group includes tocopherols and tocotrienols. An antioxidant that helps prevent damage to cells and may have a preventative role in cancer, also helps to make red blood cells.
4. Vitamin K (Menadione): All K vitamins are menadione or derivatives. Helps blood clot properly, plays key role in bone health, newborns receive vitamin K injections to prevent bleeding.

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