Section 1 Flashcards
Characteristics of the genetic code
4
- it is a triplet code (3 bases code for an amino acid)
- it is a Degenerate code (nearly all amino acids have more than one code)
- some codes don’t code for an amino acid at all and are stop codes to indicate the end of the chain (stop codon)
- some base sequences code for the same amino acid in many organisms
Define gene
A length of DNA that codes for a polypeptide (including enzymes)
Outline the steps of transcription in protein synthesis
4
1) RNA polymerase attaches to the DNA and breaks the hydrogen bonds between the bases and the DNA uncoils at the point of the new to be expressed. One of the strands of the DNA is used as a template to make an mRNA copy.
2) complementary mRNA is formed as the RNA polymerase lines up free RNA nucleotides alongside the template strand. U replaces T. Once the RNA nucleotides have paired up with DNA stand they (RNA nucleotides) join together.
3) RNA polymerase moves down the DNA strand assembling the RNA nucleotides and then the DNA strands reform into a double helix and hydrogen bonds reform
4) mRNA leaves the nucleus through the envelope pore once the RNA polymerase reaches a stop codon in the DNA sequence. The mRNA attaches to a ribosome in the cytoplasm.
Outline the steps of amino acid activation in protein synthesis.
1) tRNA passes out of the nucleus and has 3 exposed bases where amino acids bind to one end and an anticodon on the other.
2) enzymes in the cytoplasm attach amino acids to their specific tRNA molecule
Outline the steps of translation in protein synthesis
1) the mRNA attaches itself to a ribosome and tRNA molecules carry amino acids to the ribosome
2) a tRNA molecule with an anticodon complementary to the codon of AUG on the mRNA attaches to the mRNA by complementary base pairing. A second tRNA attaches in the same way.
3) the 2 amino acids attached to the 2 tRNA molecules inside the ribosome join together by a peptide bond. The first tRNA molecule moves away leaving it’s amino acid behind.
4) the third tRNA molecule binds to the next codon on the mRNA. It’s amino acid binds by peptide bonds to the first two amino acids and the second tRNA moves away producing a polypeptide chain until there’s a stop codon on the mRNA molecule.
5) the polypeptide chain then moves away from the ribosome and translation is complete
How is protein synthesis different in prokaryotes
In prokaryotes the DNA is not inside the nucleus and translation happens as soon as mRNA has been made
What are the 2 main classes of mutations and how do the occur
1) substitutions - 1 base pair replaces another in the triplet code meaning the triplet code is now different.
2) insertions and deletions - one or more base pairs in the triplet code are inserted or deleted from the length of DNA resulting in a frame shift and triplet codes are now different so May code for a different amino acid.
How can mutations have a beneficial or disadvantageous effect?
The particular environment may make the mutated phenotype disadvantageous E.G not being able to taste a substance which in large quantities can be poisonous.
However a change in environment may favour the mutated phenotype eg dark skin in Africa
How can mutations have a neutral effect
A mutation could occur in a non-coding region of DNA.
A silent mutation could occur where the triplet code changes but still produces the same amino acid as the amino acid has more than one code to code it.
A new triplet code could code for a different amino acid but that amino acid could still make the same protein.
Changed characteristics may give no particular advantage or disadvantage
What type of enzymes are found in low quantities unless when a certain substance is present when their numbers dramatically increase.
Inducers
Why aren’t all enzymes produced at a constant rate?
When some aren’t needed (for X particular sister ate not present) it wastes energy
Describe how the lac operon works when lactose is absent
1) regulatory gene is expressed and the repressor protein is synthesised
2) the repressor protein binds to the operator region and in doing so converts part of the promotor region where RNA polymerase normally attaches.
3) because RNA polymerase cannot bind to the promotor region it cannot transcribe the structural genes so no B-galactosidase or lactose permease is synthesised so no energy is wasted.
Describe how the lac operon works when lactose is present and glucose is absent
1) lactose molecules bind to the repressor protein in the active site, this changes the shape of the repressor protein so that it cannot bind to the operator region and promotor region.
2) RNA polymerase can now bind to the promoter region and B-galactosidase and lactose permease is synthesised.
B - galactosidase catalyses the hydrolysis (breaking down) of lactose into glucose and galactose.
Lactose permease allows more lactose to be broken down.
How is the feel openly of body plans controlled?
By Homeobox genes:
- Maternal-effect genes determine the polarity (which end is the head an which end is the tail)
- segmentation genes specify the polarity of each segment
- homeotic selector genes specify the identity of each segment and direct the development of individual body segments.
Why are homeobox genes similar in plants, animals and fungi?
The homeobox sequence of bases in DNA that codes for about 60 amino acids in proteins that bind to DNA to regulate transcription by turning genes on and off in specific patterns.
This gives basic pattern to the body.
All the same as they all have the same function of coding for transcription factors
