Module 2.3 - Nucleic Acids Flashcards
(MA) Why is DNA replication known as semi-conservative replication?
2 identical DNA molecules are made each with 1 strand from the original molecule (the conserved strand) which acted as a template strand and one new strand
(MA) What is the structure of a DNA nucleotide?
- one phosphate group
- one nitrogenous base (A, C, G, T)
- both joined to one deoxyribose pentose sugar
- joined with one covalent bond each
(MA) What is the structure of an RNA nucleotide?
- one phosphate group
- one nitrogenous base (A, C, G, U)
- both joined to one ribose pentose sugar
- joined with one covalent bond each
(MA) What is the difference between the structures of DNA and RNA?
- RNA has a ribose sugar instead of deoxyribose sugar
- RNA has uracil instead of thymine
- RNA is single stranded rather than double stranded
- RNA is a smaller molecule
(MA) What is the structure of a nucleotide chain (e.g. one backbone of DNA)?
- 2 nucleotides are joined together with a covalent bond
- between the phosphate group of 1 + the pentose sugar of another
- the backbone of a nucleic acid is a sugar phosphate backbone
(MA) How do two nucleotide chains in DNA bond together (e.g. in DNA double helix)?
- hydrogen bonds between the bases
- complementary base pairing
- purine to pyrimidine
- A to T and G to C
- 2 H bonds between A and T
- 3 H bonds between C and G
(MA) How does DNA replicate?
- double helix untwisted
- DNA unzipped when helicase enzymes break the H bonds between base pairs
- both strands act as a template for free DNA nucleotides to align + complementary base pair (C-G and A-T)
- H bonds between bases reform
- DNA polymerase joins the sugar phosphate backbones together on the new strands with covalent bonds
- the molecules twist back into a double helix
- there are now 2 identical DNA molecules
What are the two main nucleic acids?
- DNA
- RNA
What are the monomers of nucleic acids?
nucleotides
Describe the structure of a nucleotide.
- phosphate group
- pentose sugar (ribose/deoxyribose)
- nitrogenous base
How are nucleotides joined together?
With a covalent bond in a condensation reaction (between pentose sugar and phosphate group)
Which are larger: purines or pyrimidines?
Purines
Which of the nitrogenous bases are purines?
- adenine
- guanine
Which of the nitrogenous bases are pyrimidines?
- thymine
- uracil
- cytosine
What to purines bind to?
Pyrimidines
How would you describe the backbone of a nucleic acid?
sugar phosphate backbone
How many hydrogen bonds are there between cytosine and guanine?
3
How many hydrogen bonds are there between adenine and thymine?
2
What is a DNA molecule made up of?
2 polynucleotides forming a stable double helix structure
Suggest why purines always bonds to pyrimidines.
So the sugar phosphate backbones are always the same distance apart
Describe the semi-conservative replication of DNA.
- double helix untwisted
- DNA unzipped when helicase enzymes break H bonds between base pairs
- both strands act as a template for free DNA nucleotides to align + synthesis a new strand (in the 5’ to 3’ direction) and form complementary base pairs (A-T, G-C)
- H bonds between bases reform
- DNA polymerase joins the sugar phosphate backbones together on the new strands w covalent bonds
- molecules twist back into a double helix. Now 2 identical DNA molecules
Describe protein synthesis.
- cell requires a particular protein
- gene coding for the protein is expose by splitting H bonds holding double helix together in that region
- free RNA molecules bind to their complementary expose pairs
- sugar phosphate backbone seals to form an mRNA strand (RNA polymerase) - this is the copy of DNA for the particular protein that’s needed
- DNA is the template strand + RNA is the coding strand
- mRNA strand peels away from DNA + leaves nucleus via nuclear pore
- mRNA attaches to a ribosome
- tRNA molecules have an anticodon (3 bases) which attach to mRNA. tRNA also has a specific amino acid group at the opposite end of the molecule to the anticodon
- amino acids are close enough together for a peptide bond to form between them, thus forming a protein with a specific primary structure
What is RNA polymerase’s role?
Catalyses the formation of H bonds between RNA molecules
What is mRNA’s role?
the DNA strand which leaves the nucleus + moves towards the ribosome
What is rRNA’s role?
associates w proteins to form the ribosome. This moves along the mRNA molecule to assemble amino acids
What is tRNA’s role?
the tRNA molecule which brings the amino acid to the ribosome
Compare the structure of ATP and a DNA nucleotide.
- both have a ribose sugar
- both have a nitrogenous base (ATP it’s always adenine)
- both have a phosphate group (ATP has 3)
- become phosphorylated nucleotides when they contain more than 1 phosphate group (ATP)
Give an example of when nucleotides can be used as coenzymes.
- adenine nucleotides are components of NADP coenzyme (nicotinamide adenine dinucleotide phosphate)
- used in photosynthesis
- NAD (nicotinamide adenine dinucleotide) is a coenzyme in respiration
- FAD (flavine adenine dinucleotide) + coenzyme A (respiration)
Where is DNA found and what does it do?
- nuclei of all eukaryotic cells
- cytoplasm of prokaryotic cells
- also inside some types of virus
- hereditary material _ carries coded instructions used in development/functioning of living organisms
- important macromolecules that make up the structure of living organisms (others being carbohydrates, lipids, proteins)
Describe the structure of a DNA molecule.
- polymer: made up of many repeating monomeric units: nucleotides
- consists os 2 polynucleotide strands
- 2 strands run in opposite directions so are described as antiparallel
- nucleotide: phosphate group, deoxyribose, nitrogenous base
- covalent bond between sugar + phosphate group: phosphodiester bond. Broken when nucleotides break down + are formed when polynucleotides are synthesised
- long so can carry lots of encoded genetic information
How many rings is a purine made up of?
2
How many rings is a pyrimidine made up of?
1
Why is a DNA molecule described as antiparallel?
- upright part of large DNA molecule resembling ladder formed by sugar phosphate backbones of antiparallel polynucleotide strands
- ‘opposite directions’ of the 2 strands refers to direction the 3rd + 5th carbon molecules on the 5 carbon sugar, deoxyribose, are facing
- 5’ end of the molecule is where phosphate group is attached to 5h C atom of deoxyribose sugar
- 3’ end where phosphate group attached to 3rd C atom of deoxyribose sugar
- rungs of the ladder consist of the complementary base pairs, joined by H bonds
- molecule is v stable + integrity of the coded info within base sequences is protected
How is DNA organised in eukaryotic cells?
- majority of DNA content, or genome, is in the nucleus
- each large molecule of DNA is tightly wound around special histone proteins into chromosomes. Each chromosome is therefore 1 large molecule of DNA
- loop of DNA, without histone proteins, inside mitochondria + chloroplasts
How is DNA organised in prokaryotic cells?
- DNA is in a loop + is within cytoplasm, not enclosed in a nucleus
- not wound around histone proteins + described as naked
Which stage of the cell cycle does DNA replication take place in?
interphase
Describe the semi-conservative replication of a DNA molecule.
- unwinds: double helix untwisted, catalysed by gyrase enzyme
- unzips: H bonds between bases break, catalysed by DNA helicase, resulting in 2 single strands of DNA w exposed nucleotide bases
- free phosphorylated nucleotides, present in nucleoplasm within nucleus, are bonded to exposed bases, following complementary base pairing rules
- DNA polymerase enzyme catalyses addition of new nucleotide bases (5’ to 3’ direction) to single strands of DNA; uses each single strand of unzipped DNA as a template
- leading strand is synthesised continuously, whereas lagging strand is in fragments (discontinuous) that are later joined catalysed by ligase enzymes
- hydrolysis of activated nucleotides, to release extra phosphate groups, supplies energy to make phosphodiester bonds between sugar + phosphate
Describe the structure of RNA.
- ribose sugar
- nitrogenous base uracil
- polynucleotide chain is usually single stranded
- polynucleotide chain is shorter
- there are 3 forms of RNA: mRNA, tRNA, rRNA
Give some examples of why primary structure is important for the shape of the protein.
- shape of active site of enzyme must be complementary to substrate
- part of antibody molecule complementary to antigens on surface of an invading pathogen
- receptor on cell membrane must have complementary shape to shape of cell-signalling molecule e.g. hormone/drug, that it must detect
- ion channel protein: hydrophilic amino acids lining inside of channel + lipophilic amino acids on outside portion next to lipid bilayer
Why is the genetic code universal?
In almost all living organisms the same triplet of DNA bases codes for the same amino acids
Why is the genetic code described as degenerate?
For all amino acids (except methionine + tryptophan) there’s more than one base triplet. This may reduce effect of point mutations, as change in one base of the triplet could produce another base triplet that still codes for the same amino acid
Why is the genetic code described as non-overlapping?
-it’s read starting from a fixed point in groups of 3 bases. If a base is added/deleted then it causes a frame shift, as every base triplet after that, and hence every amino acid coded for, has changed
What are ribosomes made up of and what ion helps to bind these 2 subunits together?
- rRNA + protein
- magnesium ions
Describe transcription.
- tRNA molecules bring amino acids + find their place when anticodon binds by temp H bonds to complementary codon on mRNA
- as ribosome moves along length of mRNA, reads the code, + when 2 amino acids are adjacent to each other a peptide bond forms between them
- energy (ATP) needed for polypeptide synthesis
- amino acid sequence for polypeptide determined by sequence of triplets of nucleotide bases on length of DNA: gene
- after polypeptide assembled, mRNA breaks down. Component molecules can be recycled into new lengths of mRNA w different codon sequences
- newly synthesised polypeptide is helped, by chaperone proteins in the cell, to fold correctly into its 3D shape or tertiary structure, in order to carry out its function
