Chapter 3: Nucleic Acids Flashcards
Nucleotide structure
Many nucleotide monomers link to form a nucleic acid polymer (contain C, H, O, N and P)
Made of
- a pentose monosaccharide sugar with 5 C atoms (ribose in RNA, Deoxyribose in DNA)
- a phosphate group (PO4-3) inorganic, negatively charged
- a nitrogenous base (complex organic molecule containing N and 1/2 C rings)
Polynucleotides
Synthesis:
Condensation reactions between adjacent nucleotides where the phosphate group at 5’ forms a phosphodiester bond with the OH group at 3’ and water molecules are released
Breakdown:
Hydrolysis reactions where water is added to break the phosphodiester bonds and individual nucleotides are released
Nitrogenous bases
Pyrimidines:
Group of smaller nitrogenous bases, containing single carbon ring structures
E.g. C and T
Purines:
Group of larger nitrogenous bases, containing double carbon ring structures
E.g. A and G
ATP structure
Energy transfer in cells for synthesis, movement and transport
Made of
- adenine (a nitrogenous base, purine)
- a pentose monosaccharide sugar with 5 C atoms
- three phosphate groups (PO4-3)
Energy release from ATP
Hydrolysis reaction:
ATP + H2O → ADP + Pi + energy
ATP hydrolysed to ADP and a phosphate ion to release energy
Phosphate bonds in ATP are unstable so cannot store energy long-term
Good immediate energy store
Created by energy released during cellular respiration when breaking down long-term energy stores (e.g. fats, carbohydrates)
ADP
Adenosine diphosphate
Used to make ATP in phosphorylation process
- reattach phosphate ion to ADP molecule in a condensation reaction
Condensation and hydrolysation is reversible - recycling system
Photophosphorylation:
- in the chlorophyll during photosynthesis
- uses light
Oxidative phosphorylation:
- in the mitochondria during the electron transport chain
- part of respiration
Substrate-level phosphorylation:
- phosphate ions are transferred from donor molecules to ADP
Complementary base pairing
Nitrogenous bases bind specially where one purine binds to one pyrimidine
Adenine forms 2 hydrogen bonds with thymine
Cytosine forms 3 hydrogen bonds with guanine
There is always equal amounts of A=T and C=G
DNA Double Helix
Two polynucleotide strands held together by H bonds between bases
Coil into a double helix shape
Each strand has a phosphate group at the 5’ end and an OH group at the 3’ end
Antiparallel strands run in opposite directions
Semi conservative replication
In DNA replication the double helix separates and each strand serves as a template spike a new double-stranded DNA molecule
Each new molecule consists of one original DNA strand and one new strand
DNA Helicase
Enzyme that separates the 2 strands in the double helix
Travels along the phosphate sugar backbone and catalyses reactions that break down the hydrogen bonds between nitrogenous bases
DNA Polymerase
Enzyme that joins free nucleotides to their complementary bases
Travels along the template strand from the 3’ end to the 5’ end and catalyses the formation of phosphodiester bonds between nucleotides
Leading strand unzipped from the 3’ can undergo continuous replication
Lagging strand unzipped from the 5’ has to wait until a section has been unzipped and then work backwards along the strand - undergoes discontinuous replication
- DNA is produced in sections called Okazaki fragments that need to be joined
Mutation
Sequence of bases in DNA important for genetic code - DNA replication ensures that 2 daughter cells are genetically identical to parent cell
Errors in replication when sequence of bases are not exactly matched can lead to a change in sequence of bases - mutation
Occurs randomly and spontaneously
The triplet code
A sequence of 3 bases - codon
Each codon codes for an amino acid
The sequence of DNA is read in threes, and the section of DNA that codes for an entire protein is a gene
Genetic code is universal as all living organisms use the same code
Degenerate code
64 different base codons possible (4 x 4 x 4)
Many amino acids can be specified by more than one codon - more codons than amino acids (only 29 regularly occurring amino acids)
Each codon only species one amino acid/ stop signal so there is no uncertainty
If there is a mistake in 1 base then the overall genetic code is unlikely to be affected, correct amino acid is still produced - avoids mutations
Non-overlapping
Genetic code had a start codon (AUG) to signal the start pf the sequence - ensures codons are read in the correct reading frame (read from base 1, no overlap)
Non-coding DNA acts as a buffer zone between the coding bases
Three stop codons to signal the end of a sequence
RNA polymerase
Enzyme makes a transcript of the DNA sequence
Catalyses the formation of phosphodiester bonds between the free nucleotides and the complementary bases exposed on the antisense strand to form mRNA
mRNA
Carries genetic information from the protein-coding gene in the DNA in the nucleus, to the ribosomes - site of protein synthesis
DNA is too large to leave the nucleus but mRNA is a single strand short enough to leave the nuclear envelope
Transcription
Antisense strand is used as the template as the mRNA formed will then be identical to the sense strand (from the 5’ to 3’ end) that codes for the required protein
When the DNA is unzipped, the free nucleotides align along the exposed bases on the antisense strand in complementary base pairing. RNA polymerase joins the RNA nucleotides together by reforming the phosphodiester bonds between them to make mRNA
Translation
1) mRNA associates with a site on the small subunit of a ribosome at its start codon (AUG)
2) a tRNA with the complementary anticodon (UAC) bind to the mRNA start codon and carries the specified amino acid (methionine)
3) another tRNA with the next complementary anticodon carries the specified amino acid and bonds to the codon on the mRNA. Max of 2 tRNAs can be bound at the same time.
4) ribosome moves along the mRNAk releasing the first tRNA. The second becomes the first.
5) Process repeats until stop codon is reached and polypeptide is released.
rRNA
Major component in ribosomes, important in maintaining the structural stability of protein synthesis sequence
helps mRNA bind to the right spot on the small subunit of the ribosome, so that the base sequence can be read out
Holds mRNA in position while it is translated into a sequence of amino acids
tRNA
Necessary during translation by acting as carriers to bring amino acids to the ribosome
Ensures the amino acid that is added to the chain is corresponding to the codon specified by the mRNA
Consists of a single strand of RNA folded so complementary segments stick together, forming double-stranded regions
3 bases at one end of the molecule - anticodon
Bind to complementary codon on MRNA to form the primary structure of the protein.
