Macromolecules (nucleic acid) Flashcards
Structure of nucleotide
- phosphate grp, 5-carbon sugar and nitrogenous base
- P grp and sugar bonded via phosphoester bond
- sugar and base bonded via glycosidic bond
different types of nitrogenous base
- Purine (BIGGER): Adenine, Guanine
- Pyrimidine (SMALLER): Thymine/Uracil, Cytosine
Difference between sugar in DNA and RNA
- 2’ C in RNA is OH, as compared to DNA which has H as 2’ C
- partial (-) charge in OH repels partial (-) charge of P
- thus preventing RNA chain from coiling in as tight a helix as in DNA
- thus RNA is more susceptible to chemical and enzyme degradation
How nucleotides are joined tgt to form polynucleotide
via condensation reaction
between 5’-phosphate grp of one nucleotide
and 3’-OH grp of another
to form phosphodiester bond
Structure of DNA
- consists of 2 polynucleotide strands/chains
(- each strand forms a right-handed helix) - which coil around each other to form a double helix
- one full DNA helix turn comprises 10 base pairs
- there is presence of major and minor groves along the length of DNA
Reasons DNA is a device for stable storage of genetic information (resistant to mutations)
- extensive H bonds between base pairs
- hydrophobic interactions between stacked base pairs
=> stabilise the structure of the double helix - hydrophobic nitrogenous bases safely tucked inside double helix (while hydrophilic phosphate grps are projected outside)
/ exposure to outside influences of only sugar-phosphate bb
=> protection of nitrogenous bases from degradation - complementary base pairs between 2 strands
=> each strand serves a template to repair any DNA damage - (Eukaryotes) double helix tightly wound around histones to form nucleosome
-> folded into highly compact chromosome
=> DNA protected from thermal and physical damage
reasons for complementary base pairing between DNA strands
- steric restrictions
- dna has regular helical structure
-> double helix has uniform diameter of 2nm
-> must always pair 1 purine to 1 pyrimidine - H bond factors
- each nitrogenous base has side grps that can form H bonds with its appropriate partners
- A with T (2 bonds),
C with G (3 bonds)
significance of complementary base pairing between DNA strands
- maintains integrity of DNA
- since base seq on 1 strand dictates base seq of other strand,
- thus making genetic info redundant - DNA rep
- where both parental DNA strands separate and act as templates for synthesis of daughter DNA strands
gen 1 means after 1st replication
evidence for semi-conservative dna rep
- via density-gradient centrifugation
- gen 1: 100% of DNA mol are hybrid, resulting in only 1 band
- gen 2: 50% of DNA mol are hybrid and 50% are light, resulting in 2 separate bands
gen 1 means after 1st replication
evidence for conservative rep
- via density-gradient dna centrifugation
- gen 1: both light and heavy DNA molecules, resulting in 2 separate bands
gen 1 means after 1st replication
evidence for dispersive dna rep
- via density-gradient dna centrifugation
- gen 2: only hybrid DNA molecules present, resulting in only 1 band
1st level of condensation of packaging of DNA in cell
- DNA is coiled around histone proteins, forming nucleosome core
- histones have (+)-charged residues, which form ionic bonds with (-)-charged sugar-phosphate bb of DNA
- nucleosome cores, together with linker DNA, forms 10nm chromatin fibre
2nd level of condensation of packaging of DNA in cell
- DNA is further coiled to produce 30-nm solenoid
- histone H1 and linker DNA are involved in coiling
3rd level of condensation of packaging of DNA in cell
- scaffold proteins are involved
- in condensing the 30-nm solenoid to form looped domains
- which further coil to produce the 1400-nm condensed chromosome
Advantages of chromatin being dynamic molecule
- When condensed …
- more compact to fit into the nucleus
- has ability to save space
- helps to maintain the integrity of DNA - regulation of gene accessibility
- ability to regulate gene accessibility
- leading to differential gene expression
- e.g. when chromatin is organised as euchromatin (diffused form), available for transcription
- e.g. when chromatin is organised as heterochromatin (highly condensed form), transcriptionally inactive - regulation of cell cycle
- condensation of chromatin to discret chromosomes so that DNA does not get entangles and break during separation at anaphase
- uncondensed chromatin allows for ease of DNA rep during S-phase of interphase