1
Q
Nucleic acids?
A
- ‘info carrying’ molecules
- 2 main types = RNA & DNA
- monomer: nucleotides, polymer: nucleic acids
2
Q
Structure of RNA?
A
- ribonucleic acid
- polymer of nucleotide formed of: ribose (alw) (pentose sugar), nitrogenous base & phosphate group
- bases: adenine, guanine, cytosine & Uracil
- 3 main types: messenger, ribosomal, transfer
- single stranded generally
3
Q
mRNA?
A
- a copy of 1 gene from DNA
- created in nucleus - leaves nucleus to carry copy of genetic of gene - to ribosome in cytoplasm
- DNA too large to leave nucleus (to provide genetic code to make protein) & would be at risk of damage by enzymes and so - destroying genetic code permanently
- much shorter (as only length of 1 gene)
- short lived - only temporarily needed - by time enzymes could break down - would’ve alr carried out function
- single stranded
- every 3 bases (in sequence) code for specific amino acid aka codons
4
Q
tRNA?
A
- only in cytoplasm
- single stranded but folded to create shape like cloverleaf - held in place by H bonds
- function: to attach to 1 of 20 amino acids & transfer ts to ribosome to create polypeptide chain
- specific amino acids attach to specific tRNA molecules & bring to mRNA - determined by complimentary base pairing between codon on mRNA & anticodon on tRNA
- tRNA has 3 complementary bases to codon - - how mRNA codes
5
Q
rRNA?
A
- combines w protein to make ribosomes
- makes up the bulk of ribosomes
6
Q
DNA?
A
- deoxyribonucleic acid
- double helix structure - made up of 2 polynucleotide strands
- molecules r extremely long
- monomer: nucleotide: deoxyribose (alw) (pentose sugar) (pentagon), nitrogenous base (rectangle) & phosphate group (circle)
- bases either: A,T,C or G
7
Q
Function of DNA?
A
- codes for sequence of amino acids in primary structure of a protein - which determines final 3D structure & function of protein
- essential that cells contain copy of this genetic code & that it can be passed onto new cells w/o being damaged
8
Q
Polynucleotides?
A
- polymer of nucleotides
- created via condensation reactions between deoxyribose sugar & phosphate group - forming phosphodiester bond…
- r strong covalent bonds & so - help ensure genetic code isn’t broken down
- polynucleotide has a sugar-phosphate backbone - describes the strong covalent bonds between sugar + phosphate groups that hold polymer together
9
Q
How are the 2 polynucleotide strands in DNA molecules held together?.
A
- DNA has antiparallel strands - run in opp directions
- base pairs joined tgt by H bonds - how double helix created as 2 chains twist
- bases on opposite strands pair up by complementary base pairing - C can only form H bonds w G, A can only bond w T
- A & T - form 2 H bonds between e/o
- C & G form 3 H bonds
10
Q
DNA v RNA?
A
monomers:
- DNA contains T, RNA: U instead
- DNA: pentose sugar: deoxyribose, RNA: ribose
polymers:
- DNA: much larger (cos contains all genes/entire genome), RNA: much shorter (only length of 1 gene)
- DNA: double stranded, RNA: single stranded
11
Q
How DNA structure relates to its function?
A
- Stable structure: due to sugar-phosphate backbone (held tgt by phosphodiester/covalent bonds) - protects bases (genetic code) & due to double helix (carries genetic code) & many H bonds r strong tgt
- 2 strands held tgt by H bonds - allowing strands to be separated during DNA replication & protein synthesis (for genes to be read) (by breaking H bonds)
- double stranded so replication can occur using 1 strand as template…
- weak H bonds for easy unzipping of 2 strands during replication
- large molecule - carry huge amount of genetic info
- function of gene/protein made from gene depends on base sequence
- complementary base pairing means DNA replicated accurately so - identical copies made
12
Q
DNA replication
A
- before cells divide - in interphase
- so that there’s a copy for the new cell
- semi-conservative: daughter - 1 strand from parental DNA & 1 strand newly synthesised
13
Q
Step-by-step process of semi-conservative DNA replication?
A
- enzyme: DNA helicase, causes DNA double helix to unwind and separate into two strands by breaking the hydrogen bonds between complementary base pairs, exposing the bases.
- Both separate strands then act as a template for DNA replication to occur.
- Free activated DNA nucleotides (in the nucleoplasm) are attracted to and hydrogen bond to their complementary bases on the exposed template strands by complementary base pairing.
- The newly added nucleotides are then joined together by the enzyme DNA polymerase, which catalyses the formation of phosphodiester bonds, between adjacent nucleotides.
- This results in two identical DNA molecules being produced, each of which contains one of the original DNA strands and one newly synthesised DNA strand (semi-conservative)..
14
Q
5’ to 3’?
A
- nucleotides can only be added in 5’ to 3’ prime direction cos: DNA polymerase can only attach nucleotides to OH group on 3’ carbon/added to 3’ end of growing strand
- 5’ end: start (end that stops w phosphate group)
- 3’ end: finish (end that stops w OH group)
- phosphodiester bonds link
15
Q
Background info on Meselson & Stahl?
A
- DNA bases are nitrogenous
- N has 2 isotopes - 14N (lighter) & 15N (heavier)
- Bacteria take in N isotopes to make new DNA nucleotides…
- bacteria grown in medium - will take in any N from growth medium & incorporate it into any new DNA they make during DNA replication (nucleotides)
- 14N - hv DNA only containing ts isotope & be lighter
- bacteria grown in 15N - hv DNA - heavier
16
Q
Stages if experiment?
A
- G (Generation) 0: bacteria grown in medium containing 15N - all of DNA contains 2 strands of 15N DNA - 1 band at bottom
- G1: Bacteria from stage 1 transferred & grown for 1 gen in 14N medium - 1 band at higher pos
- G2: Bacteria from end of stage 2 grown for 1 more gen in 14N medium - 2 bands (middle: 1 strand 14N & 15N, higher: 2 strands 14N)
(thickness of band - proportion of N)
17
Q
How was conservative disproved?
A
if conservative: expect 2 bands - 1 band: at top containing 2 14N strands & 1 band at bottom containing 2 15N BUT…
- found 100% of DNA in middle
18
Q
How could they tell what Nitrogen isotope the DNA contained when extracted from the bacteria?
A
- extracted DNA spun in centrifuge in solution of cesium chloride
- when spun: DNA molecules move in test based on density/mass
- more dense molecules settle further towards bottom of test tube & vice versa
- results from centrifugation then analysed
