Nucleotides & nucleic acids Flashcards
What are the building blocks of DNA and RNA?
Nucleic acids (nucleotides)
Describe the structure of a nucleic acid
- Inorganic phosphate
- Ribose sugar
- Deoxyribose in DNA
- Ribose in RNA
- Nitrogenous base
What type of bond forms between nucleic acids?
Phosphodiester bond
List the nitrogenous bases
- A= adenine
- T = thymine (DNA only)
- C = cytosine
- G = guanine
- U = uracil (RNA only)
Describe the structure of ATP
- 3 inorganic phosphate groups
- Ribose sugar
- Adenine base
Why do cells require energy?
- Synthesis of large molecules
- Transport
- Movement
Why is ATP a universal energy currency?
Used for energy transfer in all organisms
Explain why the properties of ATP make it a good energy source
Small
- Moves into and out of cells easily
Water soluble
- Metabolic process occur in aqueous environments (e.g. cytoplasm)
Intermediate bond strength
- Little energy wasted as heat
Releases small quantities of energy
- Quantities suitable for cellular needs, so little energy wasted as heat
Easily regenerated
- Can be recharged with energy
What is produced when one phosphate bond of ATP is hydrolysed?
ADP + Pi
What type of bond forms between adjacent nucleotides?
Phosphodiester bond
What type of reaction causes phosphodiester bonds to form?
Condensation reaction
What type of reaction causes phosphodiester bonds to break?
Hydrolysis reaction
Describe the structure of DNA
- Double helix
- Long
- Two antiparallel strands of nucleotides
- Nucleotides consist of a phosphate, deoxyribose (pentose sugar), base (A, T, C or G)
- Hydrogen bonding between complementary base pairs (A-T, C-G)
Describe the structure of RNA
- Single stranded
- Short
- Nucleotides consist of a phosphate, ribose (pentose sugar), base (A, U, C or G)
Describe the complementary base pairing rules for DNA
- A-T (2 hydrogen bonds)
- C-G (3 hydrogen bonds)
Describe the complementary base pairing rules for RNA
- A-U (2 hydrogen bonds)
- C-G (3 hydrogen bonds)
Outline the bonding between DNA nucleotides
- Hydrogen bonds between complementary bases on opposite strands
- Covalent (phosphodiester) bonds between deoxyribose sugar and phosphate
Describe how the structure of DNA makes it suitable for its role
Polymer
- contains a lot of information
Hydrogen bonds
- easy to break and separate strands
Double stranded
- each strand acts as a template for replication
Antiparallel strands
- Allow double helix to twist and provide compact shape
Complementary base pairing
- allows DNA to be replicated without error, reduces frequency of mutations
Describe how to extract a pure sample of DNA from plant cells
1) Grind up cells to break cell walls
2) Mix sample with detergent
- Breaks down cell membrane
3) Add salt
- Breaks hydrogen bonds between DNA and water molecules
4) Add protease enzyme
- Breaks down histones
5) Add alcohol
- Causes DNA to form precipitate
- DNA will form white precipitate between sample and alcohol
What is semi-conservative replication?
- Mechanism by which DNA is copied
- Each molecule formed has one new strand and one from parent molecule
How is semi-conservative DNA replication carried out?
- DNA helicase unwinds DNA double helix
- Separates strands by breaking hydrogen bonds between bases
- Free DNA nucleotides in nucleus are assembled on each of the parent strands
- Nucleotides complementary base pair with exposed DNA strands
- Hydrogen bonds formed between complementary bases (A-T and C-G)
- Both strands act as template
- DNA polymerase forms phosphodiester bonds between adjacent nucleotides
- New DNA molecules rewind into double helices
- Two new DNA strands are identical to template strands due to complementary base pairing
- Each new DNA molecule has 1 old strand and 1 new strand
Outline the role of helicase in DNA replication
Helicase unwinds double strand and separates strands by breaking hydrogen bonds
Outline the role of DNA polymerase in DNA replication
DNA polymerase forms covalent bonds between nucleotides to form new strand of DNA
Describe the Meselson-Stahl experiment for DNA replication
- E. coli grown in a 15N medium until all nitrogen contained in DNA was 15N (heavy)
- Sample of DNA isolated by centrifuge
- Bacteria moved to 14N medium (light)
- DNA isolated by centrifuge after 1, 2 and 3 replication cycles
- After one replication cycle, the DNA was all of intermediate density
- After two replication cycles, two bands of DNA were seen, one of intermediate
density and one of light density - After three replication cycles, the lighter density band contained more DNA
Explain why the results from the Meselson-Stahl experiment proved that DNA was replicated in a semi-conservative manner
- After one replication cycle, the DNA was all of intermediate density
- Consistent with semi-conservative replication model, which predicts all DNA
molecules will consist of one 15N-labeled DNA strand and one 14N-labeled DNA
strand - Rules out conservative replication model, which predicts both heavy DNA and light
DNA will be present, but no intermediate density will be present - After two replication cycles, two bands of DNA were seen, one of intermediate density and one of light density
- Consistent with the semi-conservative model: half should be intermediate density
DNA and half should be light density DNA - Rules out dispersive replication model - predicts after one replication cycle, the
density of all DNA molecules will gradually become lower, so no intermediate
density DNA should remain after second replication cycle
What is the role of DNA?
- Stores all the organism’s genetic information
- Codes for sequences of amino acids
- Called the genetic code
Define gene
- A section of DNA which codes for a particular sequence of amino acids
- Heritable factor
- Occupies a specific position on a chromosome (locus)
What are ‘coding sequences’?
- Sections of DNA that code for proteins
- i.e. genes
Describe what is meant by the universality of the genetic code
- Same genetic code is found in all organisms
- Each codon in the mRNA is translated to the same amino acid
Define mutation
Random and spontaneous change in the base sequence of a gene
Define codon
3 bases on mRNA that correspond to one amino acid on the polypeptide
Define anticodon
3 bases on tRNA which are complementary to the mRNA codon
If there are 4 RNA bases (A, U, C and G), and 3 bases per codon, how many different codons is it possible to make?
4*3 = 64
Why are there only 20 amino acids coded for by the 64 possible codons?
Genetic code is degenerate
- Multiple codons code for same amino acid (e.g. AAA and AAG both code for lysine)
Some codons are ‘stop’ codons - do not code for an amino acid
What is the benefit of a degenerate genetic code?
- Helps protect against negative effects from mutations by having multiple codons code for
same amino acid
What is the benefit of the DNA code being non-overlapping?
- DNA can be read from base 1
- Ensures correct RNA codons match with correct bases on DNA
Define transcription
- Synthesis of mRNA copied from the DNA base sequence
- Carried out by RNA polymerase
- Occurs in nucleus
Describe the process of transcription
- Occurs in nucleus - produces single stranded mRNA molecule
- DNA helicase unwinds and unzips double helix, breaking hydrogen bonds
- One strand of exposed DNA acts as template (antisense strand)
- RNA nucleotides align with exposed DNA bases by complementary base pairing
- Adenine (DNA) - Uracil (RNA)
- Cytosine - Guanine
- Thymine (DNA) - Adenine (RNA)
- RNA polymerase links RNA nucleotides together
- Phosphodiester bonds form between nucleotides
- RNA strand separates and leaves nucleus through nuclear pores as mRNA
Distinguish between the sense and antisense strands of DNA during transcription
- Only the antisense strand is transcribed to mRNA
- Sense strand is not transcribed
- Has the same base sequence as mRNA (with thymine instead of uracil)
At which end of the RNA molecule is the next RNA nucleotide added?
3’ (strand grows 5’ to 3’)
Define translation
- Synthesis of polypeptides
- Occurs on ribosomes
Describe the process of translation
- Occurs on ribosomes and is synthesis of polypeptides
- mRNA binds to ribosome
- tRNA binds to mRNA at site where its anticodon corresponds to complementary codon on
mRNA - Each tRNA molecule brings an amino acid that is specific to tRNA’s anticodon
- Peptide bonds form between amino acids
- Ribosome moves along mRNA to elongate polypeptide chain
- Once tRNA has delivered its amino acid, it detaches from ribosome and next tRNA attaches
- Polypeptide chain is released when ‘stop’ codon is reached
What do DNA replication, transcription and translation have in common?
Complementary base pairing
Outline how translation depends on complementary base pairing
- Translation converts a sequence of mRNA codons to a sequence of amino acids
- Triplet of bases on tRNAs pair with complementary triplet of bases on mRNA
- Base pairing occurs when A pairs with U and G pairs with C
- Specific amino acids are attached to specific tRNA
- mRNA has codons and tRNA has anticodons
What is the difference in function between free ribosomes and bound ribosomes?
- Free ribosomes make proteins for use within the cell
- Bound ribosomes make proteins for secretion or use in lysosomes
Where are ribosomes found?
- Bound to RER (rough endoplasmic reticulum)
- Free in cytoplasm
Describe differences in ribosomes in eukaryotes and prokaryotes
- 80S type in eukaryotes
- 70S type in prokaryotes
What are ribosomes made out of?
- Protein - to stabilise
- rRNA (ribosomal RNA) - enzyme activity
