Genetic Control of Protein Structure and Function Flashcards
mRNA
mRNA is a long strand that is arranged in a single helix. Mirror copy of part of one of DNA’s two strands. mRNA leaves the nucleus via nuclear pores in the nuclear envelope and enter the cytoplasm where it relays the information to ribosomes. Acts as a template upon which proteins are built. It possesses the correct sequences of many triplets of organic bases that code for specific polypeptides. It’s easily broken down so exists only when it’s needed to manufacture a given protein.
tRNA
tRNA is a single-stranded chain folded into a clover leaf shape, with one end of the chain extending beyond the other. This is where amino acids attach to. There are several types of tRNA and each can carry a single amino acid. The opposite end of the tRNA molecule is a sequence of 3 other organic bases called the anticodon (complimentary to the codon). tRNA lines up amino acids on the mRNA template during protein synthesis.
codon
Sequence of triplet bases on mRNA that codes for single amino acid.
anticodon
Sequence of triplet bases on tRNA that is complimentary to the codon.
degenerate code
Most amino acids have more than one codon.
stop-codons
3 codons do not code for any amino acid. They mark the end of a polypeptide chain.
non-overlapping
Each base in the sequence is read only once.
universal code
Same codon codes for the same amino acid in all organisms.
transcription
A complementary section of part of DNA sequence is made in the form of pre-mRNA
translation
mRNA is used as a template to which complementary tRNA molecules attach and the amino acids they carry are linked dot form a polypeptide
Introns
Non-coding sections of DNA
Exons
Sections of DNA that code for proteins
Splicing
Removing introns and re-combining exons
Outline Translation
- A ribosome becomes attached to the starting codon on mRNA
- The tRNA molecule with complimentary anticodon sequence moves to the ribosome and pairs up with the sequence on the mRNA, carrying with it a specific amino acid.
- The ribosome moves along the mRNA, bringing together two corresponding tRNA molecules at any one time, each paring up with the corresponding two codons on the mRNA
- Amino acids on the tRNA are joined together by a peptide bond catalysed by an enzyme and ATP
- As the ribosome moves on, the first tRNA is released from its amino acid and is free to collect another amino acid from the amino acid pool in the cell
- The synthesis of the polypeptide chain continues until a stop codon is reached. At this point, the ribosome, mRNA and last tRNA will separate and the polypeptide chain is complete.
Why is splicing important?
DNA is made of exons which code for proteins and introns which don’t. Intervening introns would interfere with the synthesis of a polypeptide. Splicing removes intervening non-function introns and join function exons together. The remaining exon sections can be rejoined in a variety of different combinations, allowing the coding of up to a dozen different proteins from a single section of DNA. Mutations affect the splicing of pre-mRNA.
Outline Transcription
- DNA helicase acts on a specific region of DNA molecule to break hydrogen bonds between bases causing strands to separate and expose the nucleotide bases in that region
- RNA polymerase moves along one of the two DNA strands known as the template strand, causing the nucleotides on this strand to join with the individual complementary nucleotides
Base pair links are formed -> Uracil instead of thymine is formed - RNA polymerase adds the nucleotides one at time to build a strand of pre-mRNA, DNA strands rejoin behind it. Only 12 base pairs on DNA are exposed at any one time.
- When RNA polymerase reaches a particular sequence of bases on the DNA, it recognises a ‘stop’ triplet code and detaches -> pre-mRNA production complete
3 differences between DNA and mRNA + tRNA
- Double polynucleotide chain (RNA = single)
- Largest molecule of the three, double-helix molecule
- Deoxyribose
3 differences between mRNA and tRNA
- mRNA = Single-helix molecule, tRNA = Clover-shaped
- mRNA is larger
- mRNA is less chemically stable
4 similarities between mRNA and tRNA
- Single polynucleotide chain
- Pentose sugar ribose
- Manufactured in nucleus, found throughout cell
- Uracil base
A nonsense mutation
Base change results in the formation of 1/3 stop codons
A mis-sense mutation
Base change results in a different amino acid being coded for
A silent mutation
Substituted base codes for the same amino acid - degenerate
Deletion/ Addition
Frameshift due to triplet code
2 Mutagenic agents
- high-energy radiation
2. chemicals that alter the DNA structure or interfere with transcription
Role of Proto-oncogenes
Stimulate cell division. Growth factors attach to a receptor protein on cell-surface membrane via relay proteins in cytoplasm, ‘switch on’ the genes necessary for DNA replication.
What happens if Proto-oncogenes mutate?
They can mutate into oncogenes which can either:
- Permanently activate the receptor protein on the cell-surface membrane so cell division is switched on even in the absence of growth factors
- Code for a growth factor that is then produced in excessive amounts, stimulating excessive cell division
Role of tumour suppressor genes
Maintains normal rates of cell division and prevents the formation of tumours.
What happens if tumour suppressor genes mutate?
- Inactivated -> most will die
2. Any that survive are able to make clones of themselves forming tumours
