Nuclaear Acids : Structure And Function Flashcards
Nucleotides vs nucleosides
Nucleotides :
- Pentose sugar
-nitrogenous base
- phosphate groups
Nucleoside :
No phosphate groups
Only pentose sugar and nitrogenous base
Nucleotides polymerise direction
5’->3’
Phosphodiester bonds between the phosphate group on carbon 5 and oh group of carbon 3
Base pairs
Hydrogen bonds
Atgc
Augc.
Structure and function of dna
Structure :
- left handed helix
Helix is stabilised by :
- hydrogen bonds between complementary base pairs
- hydrophobic interactions in base stacking
2 grooves : minor and major
- proteins can interact at groove
- they run anti parallel
Function :
- stores genetic information
- complementary base pairing nature of nucleotides allows for important processes of the cell to take place
Structure and functions of rna
Structure :
Short double helix structures formed between complementary base pairs
-augc
Stabilised by :
- hydrophobic interactions in base stacking
- hydrogen bonds in complementary base pairs
+ hydrogen bonds between bases and sugar phosphate help stabilise rna structure
Functions :
RNA : catalytic molecules
S: snRNA : mediate splicing eukaryotic gene transcript to form mature mRNA
M: mRNA : carry the info
R: rRNA : structural and functional foundation for ribosomes
T : tRNA : carry the amino acids to ribosomes for use in protein synthesis
Dna replication process (semi conservative)
- Unwinding of DNA
-Begins at Ori and is bidirectional
Pro : single , eu : multiple
-. Replication bubble is formed at ori as proteins recognise the sequence and attach to the DNA and separate the 2strands
- replication fork is owed , as parental stands unwind
2.initiation by various proteins
-helicase : unwind the dna strands to form 2 single strands that can act as templates.
-Single strand bringing proteins : bind to unwinded dna and prevent them from reassociating into a double helix
-primase bids to single strand DNA and synthesis the RNA primer (it catalyses the formation of phosphodiester bonds between ribonucleotides to form the rna primer )
-topoismerase : relieve strain caused by unwinding by breaking , swiveling , rejoining the DNA - Elongation at replication fork by DNA polymerase
RNA primer provides a free 3 oh group for dna polymerase to recognises and start dna synthesis.
Synthesis of dNA involves condensation reaction that forms a phosphodiester bond between the monomer
A)leading strand : sythesised continuously towards the replication fork
B ) lagging strand : synthesised discontinuously and elongating away from the replication fork (forming Okazaki fragments )
Ligase forms phosphodiester bonds between adjacent nucleotides of each dna fragment to form a continuous strand of dna
PS . Dna polymerase have proofreading function : exonuclease ( 3’->5’) to remove mismatched base pairs - Rewind back into double helix :
Semiconservative dna relication
End replication problem
Primer of last Okazaki fragment not replaced by DNA . Will eventually be degraded
Telomerase resolves it.
Genetic code : the sequence of the 3 bases in the mRNA ( codon ) and amino acid
Characteristics
Start codon :AUG —> initiate the process of protein synthesis
Stop codon : UAG UGA UAA —> terminates elongation of the polypeptide chain
Nearly universal
Dna — 3 regions . Promoter , transcription unit , termination sequence
Promoter ; recognition site for binding of RNA polymerase
Transcription unit : aka template strand that is transcribed into mRNA
Termination unit: end unit of a gene to stop transcription
Transcription in eukaryotes
Initiation :
-General transcription factors assemble along the o outer region .
- And recruits RNA polymerase to form the transcription pre initiation complex .
- RNA polymerase unwinds the dna and starts synthesis
Elongation :
-Free ribonucloetides match up with the template DNA strand by complementary base pairs .
-Augc
-phosphodiester bonds
MRNA strand synthesised and elongated in the 5 to 3’ direction . (Read in the 3’-5’)
Termination :
RNA polymerase transcribes a sequence on the DNA called the AAUAA polyadenylation signal . It result in binding proteins to the transcription . This results in the cleavage of rna transcript downstream .
Resulting in pre-mRNA
Transcription in prokaryotes
Initiation :
-Sigma factor associates with core RNA polymerase forming RNA polymerase holoeznyme
-scans the dna until the sigma factor recognises specific sequence of the promoter and binds to it . DNA double helix at promoter unzips through breaking of hydrogen bonds between base pairs.
- only one template is used to synthesis mRNA .
Elongation : same as eukaryotes
AUGC
Phosphodiester bonds
5’->3’
Termination :
After transcribing through a termination sequence , the newly made RNA transcript dissociate from the DNA ,
Forming mature mRNA ( only in eukaryotes )
Addition of 5’ caps :
Covalent attachment of 7 methylguanosine to the 5’ end of the transcript
Function : recognition sites
Spicing of introns by snRNA and Joins exons
Addition of polyA tails : in yhe process aka polyadenylation
Endonucleas recognises the polyadenylation sequence on the mRNA and cleaves the premRNA a few nucleotides downstream of AAUAA
Function:
-export mRNA out of nucleus
Machinery for translation : tRNA
Attachment of amino acids to tRNA molecules are catalyses by aminoacyl-tRNA-synthetases
Function. :
-Bring in specific amino acids in a sequence complementary to the sequence of coo in the mRNA to the growing polypeptide .
-facilitate translation
Machine for translation : ribosomes.
During translation :
Small subunit holds the mRNA
large subunit : where the peptide on formation occurs
Large subunit have 3 sites for holding tRNA
A : holds incoming tRNA
P : holds tRNA attached to growing Le tide
E: holds outgoing tRNA without any attached Amino
Translation in prokaryotes :
Initiation. :(requires GTP)
Shine dalgarno sequence binds to the complementary sequence on rRNA of small subunit of ribosome .
Initiator tRNA binds with start codon.
Large subunit of ribosome binds to initiator tRNA such so that in is in the P site .forming the translation initiation complex .
Elongation :
- anticodon of incoming aminoacyl tRNA complementary base pairs wth mRNA codon in A site by forming hydrogen bonds .
- peptide transferase in large ribosomal subunit catalyses peptide bond formation between amino acid carried by tRNA in A site and methionine/ amino acid carried by tRNA in the P site .
- ribosome shifts one codon down mRNA in 5’ to 3’ direction .
Peptidyl-tRNA with growing polypeptide is translocated from A site to P site.empty A site is ready to receive the next incoming aminoacyl tRNA
- process continues until top codon is reached
Termination :
When stop codon UAA,UAG,UGA reaches A site , release factor enter , hydrolysing the bond between polyp chain. and tRNA at Psite .
