How does DNA code for proteins?

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Created by sophie.holland96 over 7 years ago


Genetic Code, Transcription and Translation and Mutations

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What is a gene? A length of DNA that codes for one or more polypeptides, including enzymes.
What is meant by the term GENETIC CODE? The series of bases on a gene provides a code. 3 nucleotide bases code for one amino acid - this is why the genetic code is described as a triplet code. It is degenerate - all amino acids (other than methionine) have more than one code. It is a widespread code but not universal.
Define GENOME The entire DNA sequence of an organism
Genes code for polypeptides, such as ... Structural Proteins (collagen/keratin), Enzymes, Haemoglobin, Immunoglobulins (antibodies), Cell surface receptors, Antigens, Actin and Myosin in muscle cells, Channel proteins, Electron carriers
Protein synthesis is made up of two stages, what are they? Transcription Translation
Transcription: Where does this occur? In the nucleus
Transcription: What is the purpose of transcription? To create strand mRNA that is complementary to the template strand and is therefore a copy of the coding strand.
What happens in transcription? - The gene unwinds and unzips (helicase enzyme). Breaking of hydrogen bonds. - Activated RNA nucleotides hydrogen bond to their exposed complementary bases on the template strand. This requires the enzyme RNA polymerase. - The phosphate groups on the RNA nucleotides are released, thus releasing energy used to bond adjacent (next to) nucleotides. - The mRNA strand is released from the DNA and passes out of a nuclear pore, to a ribosome.
What are the rules of base pairing in DNA? A -- T (2 hydrogen bonds) C--G (3 hydrogen bonds) REMEMBER - in RNA the T is swapped for U (urasil) so U hydrogen bonds to A when making mRNA.
What is an activated RNA nucleotide? A free RNA nucleotide that has two extra phosphoryl (phosphorus) groups attached, when theses groups are released it releases energy, which is used to join adjacent nucleotides.
Translation: What is the purpose of translation? To assemble polypeptides from mRNA, at ribosomes
Translation: Where may ribosomes be found in an animal cell? Free in the cytoplasm Bound to the rough Endoplasmic Reticulum (ER)
Translation: Why is it so important that the amino acids are assembled in the correct order? Because it forms the tertiary structure, which in turn determines the tertiary structure. If the primary structure is altered then so is the tertiary structure, and the tertiary structure is what allows the protein to function properly, so if altered it will no longer function effectively (e.g. active sites on enzymes and protein channels)
Translation: Why is tRNA important? Because it carries the corresponding amino acids to the ribosome.
Translation: What are the features of tRNA which allow it to perform its function? It has 3 exposed bases on one end where an amino acid can join. At the other end it has 3 unpaired nucleotide bases which form an ANTICODON, which can bind temporarily with its complementary codon
Translation: What happens during translation? - mRNA binds to a ribosome - The first codon (3 bases) is read. - using ATP and an enzyme the tRNA with the complementary anticodon forms hydrogen bonds with its codon. The tRNA brings with it the corresponding amino acid for that codon - This happens for the second codon and a peptide bond forms between the adjacent amino acids. - the ribosome moves along reading more codons, as the ribosome moves along the tRNA can leave. - this continues until a stop codon is reached, which has no corresponding tRNA or amino acid.
What is Cyclic AMP (cAMP) and what is its importance? It is a chemical that activates proteins by altering their 3D shape, so that their shape is a better fit to their complementary molecules. (this is all the spec requires you to know about cAMP)
What is a DNA mutation? A change to genes due to changes in the nucleotide base sequences.
What are the two main classes of DNA mutation? 1. Point mutations (also called substitutions) 2. Insertion/deletion mutations
What is a point mutation? When one base pair replaces another. E.g. AAG ATC ATT --> ACG ATC ATT (The C has replaced the A in the first codon) Only one codon is affected, so point mutations usually can have less effect than deletion/insertion.
What is an insertion/deletion mutation? One or more nucleotide pairs are inserted or deleted from the length of DNA. These cause FRAMESHIFT These mutations are likely to have a greater effect on the resulting protein.
What is frameshift? When more than one codon is affected by an insertion/deletion mutation, thus affecting the whole length of DNA and having a large affect on the resulting protein.
Give some examples of genetic diseases caused by DNA mutations. Cystic Fibrosis (deletion of 3 base pairs) Sickle-cell anaemia (point mutation, substituting valine for glutamic acid) Cancers - point mutation in the growth promoting genes (protooncogenes), causing them to remain permanently switches on, resulting in a tumour. Huntington disease - triple nucleotide repeat, (type of insertion mutation) a stutter.
Define Allele An alternative version of a gene.
When may a mutation produce no change to an organism? - if the mutation is in a non-coding region of DNA - if it is a silent mutation. Although the base triplet has changes, it still codes for the same amino acid, due to the degenerate code.
Mutations can have neutral and beneficial effects. What does neutral mean in this context? Give an example of a mutation with a neutral effect It means that mutation does not give an advantage or disadvantage to the organism. E.g. being able to roll your tongue
Mutations can have neutral and beneficial effects. What does beneficial mean in the context? Give an example of a mutation with a beneficial effect. It means that the mutation gives the organism a selective advantage and it is more likely to survive and reproduce. e.g. Having pale skin in cooler climates allows you to produce more vitamin D, so that you didn't develop diseases such as rickets.
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