In this IBDP Biology topic, you will understand the formation of protein.

• A gene is a length of DNA that codes for one (or more) polypeptides.
• In the human genome, there are about 25000 genes.
• Each gene occupies a specific locus on a chromosome.
• Genes code for enzymes, they are involved in the control of all metabolic pathways and thus in synthesis of all non-protein molecules found in the cells.

• It is a triplet code ( a sequence of three nucleotide bases) for an amino acid.
• It is a degenerate code (all amino acids have more than one code.)
• Some codes don’t correspond to an amino acid, but indicate ‘stop’,
• It is widespread, but not universal.

• To be transcribed, a gene has to unwind and unzip (hydrogen bonds between bases break as the gene dips into the nucleolus).
• Activated RNA nucleotides bind to complementary bases on the template strand (catalysed by the enzyme RNA polymerase).
• The two extra phosphates are released, releasing energy for bonding adjacent nucleotides.
• The mRNA produced is complementary to that on the template strand and is therefore a copy of the base sequence on the coding strand of the length of the DNA.
• The mRNA is released from the DNA and passes out of the nucleus, through a nuclear pore, to a ribosome.

• The second stage of protein synthesis, when the amino acids are assembled into a polypeptide.
• They are assembled into the sequence dictated by the sequence of codons on the mRNA.
• The genetic code, copied from DNA to mRNA, is now translated into a sequence of amino acids. This is a polypeptide.
• It happens in ribosomes, which may be free in the cytoplasm but many are bound to the rough endoplasmic reticulum.

• Assembled in the nucleolus of eukaryote cells, from ribosomal RNA and protein.
• There is a groove in which the length of mRNA can fit. The ribosome then moves along he mRNA, reading the code and assembling the amino acids.

The sequence of amino acids;

•  forms the primary structure of a protein
•  primary structure determines the tertiary structure, allowing it to fold up into the 3d shape.
•  tertiary structure is what allows a protein to function.

• Another form of RNA, tRNA is made in the nucleus and passes into the cytoplasm.
• They are lengths of RNA folded into hairpin shape, with three exposed bases where a particular amino acid can bind.
• At the other end there are three unpaired nucleotide base, known as an anticodon. Each anticodon can bind temporarily with its complementary codon.

1. A molecule of mRNA binds to a ribosome. Two codons are attached to a small subunit of the ribosome and exposed to the large subunit. The first exposed mRNA codon is always AUG. Using ATP energy and an enzyme, a tRNA with methionine and the anticodon UAC forms hydrogen bonds with this codon.
2.  A second tRNA, bearing a different amino acid, binds to the second exposed codon with its complementary anticodon.
3.  A peptide bond forms between the two adjacent amino acids. An enzyme, present in the small ribosomal subunit, catalyses the reaction.
4.  The ribosome now moves along the mRNA, reading the next codon. A third tRNA brings another amino acid, and a peptide bond forms between it and the dipeptide. The first tRNA leaves and is able to collect and bring another of its amino acids.
5.  The polypeptide chain grows until a stop codon is reached. There are no corresponding tRNAs for those three codons, UAA, UAC or UGA, so the polypeptide chain is now complete.

A mutation is a change in the amount of, or arrangement of, the genetic material in a cell.

Chromosome mutation involves changes to parts of or whole chromosomes.

DNA mutations are changes to genes due to changes in nucleotide base sequences;

• Point mutations in which one base pair replaces enough (sustitutions)
• Insertion/deletion mutations in which one or more nucleotide pairs are inserted or deleted from a length of DNA (framshift)

If a gene is altered by change to its bases sequences, it becomes another version of the same gene, it is an allele of the gene. It may produce no change if;

• The mutation is in a non-coding region of the DNA
• It is a silent mutation. Although the base triplet has changed, it still codes for the same amino acid, so the protein is unchanged.

If the mutation does cause a change of characteristics, but this characteristic gives no effect to the organism, it is also thought as neutral.

• Early humans in Africa most certainly had dark skin.
• The pigment protected them from the harmful effects of UV light, but they could still synthesis vitamin D from the sunlight on their skin.
• Any humans who had mutations to some of the genes determining skin color, producing paler skin would have been burnt.
• As humans migrated to more temperate climates, the sunlight was not as intense enough to make vitamin D on dark skins.
• Humans with the mutation would have been able to synthesis more vitamin D.
• The environment is never static,  and when it changes, people who have a certain characteristic may be better adapted to the new environment.

This is the end of the topic

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Drafted by Eva (Biology)

Photo references:

1. https://medlineplus.gov/genetics/understanding/basics/gene/
2. https://sejkai.gitbook.io/academic/biology/central_dogma
3. https://courses.lumenlearning.com/wm-biology1/chapter/reading-steps-of-genetic-transcription/
4. https://www.youtube.com/watch?v=7R-fxGmqw-0
5. https://en.wikipedia.org/wiki/Transfer_RNA
6. https://ib.bioninja.com.au/standard-level/topic-2-molecular-biology/24-proteins/gene--polypeptide.html
7. https://ib.bioninja.com.au/standard-level/topic-3-genetics/32-chromosomes/block-mutations.html