Topic 1.3 Nucleotides Flashcards
The nucleotide
-Nucleotides are monomers of Nucleic acids (DNA and RNA)
-A nucleotide is composed of:
-a pent sugar (DNA-deoxyribose, RNA-ribose)
-an organic Nitrogenous base
-a phosphate group
Base groups: Purines
Adenine (DNA+RNA)
Guanine (DNA+RNA)
-Longer as the contain two nitrogenous rings
Base groups: Pyrimidines
Cytosine (DNA+RNA)
Thymine (DNA only)
Uracil (RNA only)
-Smaller as they only contain are nitrogenous ring
Deoxyribose Nucleic Acid
-Double helix structure
-Pentose sugar: deoxyribose
-Bases:
-adenine
-thymine
-guanine
-cytosine
-Base pairs: AT CG
-Held together by hydrogen bonds (weak but extensive)
Ribonucleic Acid
-Single stranded and linear structure
-Pentose sugar: ribose
-Bases:
-adenine
-cytosine
-guanine
-uracil
-Base pairs: AU CG
Central Dogma in genetics
DNA—>RNA—>Protein
ATP
-Adenosine triphosphate (nucleotide)
-Three phosphate groups
-ATP is made during respiration
-Condensation reaction (ATP synthase)
ATP + H2O –> ADP + Pi
-Opposite reaction: hydrolysis (ATP hydrolase)
ATP process (5)
- ATP releases small, manageable amounts so no energy is wasted
- ATP is a small and soluble molecules easily transported around the cell
- Only one bond is broken/hydrolysed to release energy, which is why energy release is immediate
- It can transfer energy to another molecule by transferring one of its phosphate groups
- ATP can’t pass out of the cell, the cell laws has an immediate supply of energy
(all cells need to respire so they can produce ATP)
ATP ⇌ ADP + Pi
(–>hydrolysis <–condensation
Semi-conservative replication (4)
- DNA helices unwinds/unzips the double stranded DNA by breaking the hydrogen bonds
- This produces two single-stranded DNA template
- New DNA nucleotides now join with their specific complimentary base on the template strand
- These are joined by DNA polymerase (has ‘proof reading’ abilities; checks for mistakes) making sure no mutation occurs
Each daughter cell has:
-one original strand (conserved)
-one new strand
A gene
A gene is a sequence of DNA base which codes for a sequence of amino acids to form a protein.
Genetic code
The sequence of triplets which codes for the sequence of amino acids
-Three bases on DNA codes for one amino acid
Genetic code rules: Universal
The same triplet/codons code for the same amino acid in all organisms.
Genetic code rules: Degenerate
More than one triplet can code for a particular amino acid.
(There are more codons than amino acids so some aminos acids are coded for by more than one codon).
Genetic code rules: Non-overlapping
Three codons are always read together in order.
Start codons
DNA: TAC
mRNA: AUG
Stop codons
DNA: ATC AGT ATT
mRNA: UAG UGA UAA
Transcription (nucleus) (6)
- DNA helicase unwinds and unzips the DNA by breaking hydrogen bonds between bases
- RNA polymerase binds to the non-coding region of DNA up from the gene to be transcribed
- RNA polymerase reaches the coding region of the gene and starts making a copy of the template strand (anti-sense strand)
- RNA nucleotides are joined by phosphodiester bonds to form pre-mRNA strand
- The RNA splicing occurs which cuts out introns and joins the exons to form mRNA
- The mRNA leaves the nucleus via a nuclear pore and enters the cytoplasm.
Translation (ribosome) (6)
- mRNA attaches to a ribosome in the cytoplasm, two codons at a time (a codon- three bases)
- A tRNA molecule with the complimentary anti-codon binds to the codon
- The amino acids join together to start forming a polypeptide by a peptide bond
- The ribosome moves down the mRNA one codon at a time and another amino acid is joined to the polypeptide
- The polypeptide then folds into shape to form a protein eg. enzyme, haemoglobin
- The protein has a specific shape determined by the order of the amino acids.
Intron
A sement of DNA or RNA molecule which doesn’t code for proteins and interrupts the sequence of genes.
Exon
A segment of DNA or RNA molecule containing information coding for a protein or peptide sequence.
Protein synthesis ‘equation’
DNA ———–> mRNA ———–> Protein
(transcription) (translation)
Post translational modification (PTM)
-When the protein has made it is not always functional; inactive
-It undergoes PTM is the Golgi body
-eg. insulin 100 amino acids undergoes extensive PTM to produce active insulin which is 51 amino acids long
Mutation: Substitution
One base is substituted for another
ie. the wrong base is inserted
(change in primary structure; change in 3D tertiary structure; different hydrogen bonding; no enzyme-substrate formed; non-functioning protein)
Mutation: Addition
An extra base is added to the sequence (frame shift)
All the triplets after the addition will shift
Produces a major change in the protein because its a totally different primary sequence.
Mutation: deletion
A base is removed from the sequence.
This also causes a frame shift.
What causes mutations?
-Random occurrence
-Ionising radiation eg. UV light, X-rays etc
-Chemicals called mutagens eg. mustard gas, chemical,ls in cigarette smoke.
Chromosomal mutation
An extra copy of a chromosome
eg. Downs syndrome- 3 chromosome 21s (an extra)
How do polynucleotide strands form?
Condensation reactions between nucleotides form strong phosphodiester bonds (sugar-phosphate backbone)
How is a new strand formed during semiconservative replication?
1) Free nucleotides from nuclear sap attach to exposed bases by complementary base pairing
2) DNA polymerase joins adjacent nucleotides on new strand in a 5’ –> 3’ direction via condensation reactions to form phosphodiester bonds
3) H bonds reform
Structure of mRNA
-Long ribose polynucleotide with sugar- phosphate backbone
-Single stranded and linear (no H bonds between complementary base pairs)
-Codon sequence is complimentary to axons of 1 gene from 1 DNA strand.
Structure of tRNA
-Single strand folded into clover shape (some paired bases)
-Anticodon on one end, amino acid binding site on the other
-
Antisense strand
Template of DNA which is transcribed.
Sense strand
Strand with the same base sequence as mRNA (but with thymine instead of uracil)
What happens after a strand of mRNA is transcribed?
-RNA polymerase detaches at terminator region
-H bonds reform and DNA unwinds
-Splicing removes introns from pre-mRNA in eukaryotic cells
-mRNA moves out of nucleus via nuclear pore and attaches to ribosome
State the role of ATP during translation
ATP hydrolysis provides energy to form peptide bonds.
What is sickle cell anaemia?
Genetic condition that results in abnormal haemoglobin.
Impaired ability to transport oxygen= rapid heart rate, fatigue dizziness.
Sickle shaped red blood cells ‘stick’ in vessels.
What causes sickle cell anaemia in humans?
Missense point mutation in gene that codes for β strand in haemoglobin.
Change in primary structure= different tertiary structure.
Abnormal haemoglobin molecules form strands that make red blood cells sickle shaped.
