Functions of Nucleic Acids Flashcards
How is DNA replicated?
- DNA unwinds
- hydrogen bonds holding complementary bases pairs break, catalysed by DNA helicase and the 2 strands separate
- each DNA strand acts as a template
- free DNA nucleotides align opposite their complementary base pairs
- DNA polymerase catalyses the condensation reaction between 2 DNA nucleotides. This occurs from the 5’ end to the 3’ end of the chain
- each new DNA molecule is made from one original template strand and one newly replicated strand - semi conservative replication
What is conservative replication?
Th parental double helix remains intact, completely conserved, and a whole new DNA molecule is made
What is semi-conservative replication?
Each strand in the parental double helix acts as a template to synthesise a new polynucleotide strand. Each new DNA molecule contains one template strand and one newly replicated strand
What is dispersive replication?
Each new DNA molecule contains fragments of the parental double helix and newly synthesised DNA
Why was the Meselson-Stahl Experiment conclusive evidence of semi-conservative replication?
- after one generation, semi-conservative replication would give one band in the middle (each DNA molecule has one ‘heavy’ strand and one ‘light strand of DNA
- after one generation, conservative replication would not give a band in the middle, only a band at the top (light) where the DNA has two ‘light’ strands of DNA made from N14, and a band at the bottom (heavy) where the DNA has 2 ‘heavy’ strands made from N15
- after one generation, dispersive replication would only produce on band, which got progressively higher in the tube
What is protein synthesis?
Occurs in two stages
- transcription
- translation
DNA = triplet
mRNA = codon
tRNA = anticodon
What is transcription?
- one strand of DNA is used as a template strand for the transcription of mRNA
- DNA helicase breaks the hydrogen bonds holding complementary holding together the base pairs in a specific region of the DNA molecule, so the two strands unwind
- RNA polymerase binds the the template strand at the beginning of the sequence to be copied
- free RNA nucleotides align opposite complementary bases on the template strand (A=U, C=G)
- RNA polymerase moves along the template strand, catalysing the addition of RNA nucleotides to each other until it reaches a stop codon
- behind the RNA polymerase, the DNA strands rewind
- mRNA leaves via a nuclear pore and travels into the cytoplasm
What is an exon?
The coding sequences which are left behind and are spliced together by ligase enzymes to form the final mature mRNA
What is an intron?
The non-coding nucleotide sequences in DNA that are removed from pre-mRNA after transcription by the enzyme endonuclease which acts on the mRNA
Prokaryotic DNA does not contain introns
How is mRNA modified after transcription before translation?
The initial mRNA molecule produced by transcription is longer than the final mRNA that is translated at he ribosome, so the pre-mRNA needs to have sequences of bases removed
Non-coding introns are removed from the pre-mRNA after transcription by endonuclease enzyme, and the remaining coding exons are spliced together by ligase enzymes to form the mature mRNA. The mature mRNA then exits via the nuclear pore to travel towards the ribosome in the cytoplasm or on the RER
How can more than one polypeptide be produced from one gene?
The exons could be spliced back together in a different order, thus coding or a different sequence of amino acids, which would result in a different polypeptide
What is tRNA?
- clover leaf shape
- carries specific amino acids to the ribosome
- anti-codon determines which amino acid the tRNA molecule will carry
- attaches to the amino acid in the cytoplasm - requires ATP and is called ‘amino acid activation’
What is a ribosome?
- made from protein and rRNA
- can be free in the cytoplasm or attached to the rough endoplasmic reticulum
- each consists of a small subunit and a large subunit
- large subunit has two attachment sites for tRNA molecules and the smaller subunit binds to the mRNA
What is initiation in translation?
- ribosome attaches to a start codon on the mRNA
- first tRNA binds to the first attachment sites, the anti-codon on the tRNA joins to the complementary codon on the mRNA by hydrogen binds to form a codon-anticodon complex
- a second tRNA forms a codon-anticodon complex at the second attachment site
What is elongation in translation?
- a ribosomal enzyme catalyses the formation of a peptide bond between the adjacent amino acids
- the first tRNA leaves site 1 and returns to the cytoplasm
- the ribosome moves down the mRNA one codon, so that the second tRNA moves from site 2 to site 1
- a new tRNA binds to site 2
What is termination in translation?
- sequence repeats until a stop codon is reached
- a ribosome-mRNA-polypeptide complex separates
What is a polysome?
When several ribosomes move down one mRNA strand. Each ribosome produces a polypeptide, so several chains are made at once
How are proteins modified post-translation?
- the sequence of codons on the mRNA is the primary polypeptide structure, which is transported to the Golgi body in vesicles for further modification
- it can be folded further or it can be chemically modified by combination with non-proteins like
— carbohydrates (glycoproteins)
— lipids (lipoproteins)
— phosphate (phospho-proteins)
What does the sequence of bases determine?
The sequence of amino acids that are joined together to form a polypeptide
Three bases code for each amino acid (triplet code)
64 possible codes, only 20 amino acids
What is the one gene-one polypeptide hypothesis?
Some proteins contain more than one polypeptide, a section of DNA that codes for a specific polypeptide is a gene
Why are there more triplet codes than there are amino acids?
Each amino acid has more than one triplet code, the code is then called degenerate. Some triplet codes do not code for amino acids, but are ‘stop codons’ and mark the end of translation
What is the genetic code universal and non overlapping?
UNIVERSAL: the same triplet codes for the same amino acid in all living organisms
NON-OVERLAPPING: each base in the sequence occurs in only one triplet
