DNA, RNA Flashcards
What are nucleotides made up of?
1) Nitrogenous bases (Adenine, Thymine/ Uracil, Cytosine, Guanine)
2) Pentose sugar
3) Phosphate group (H3PO4) - gives nucleic acid its acidic character
What are purines?
Two carbon-nitrogen rings (AG)
What are pyrimidines?
One carbon-nitrogen ring (C,T/ U)
What is the difference between the pentose sugar in DNA and RNA?
1) DNA: deoxyribose sugar (H at C2)
2) RNA: ribose sugar (OH at C2)
**Significance: DNA is more stable, less easily broken down compared to RNA
How are nucleotides formed?
1) 3 components link up to form a nucleotide via 2 condensation reactions (2 molecules of water removed) :
2) 1st condensation reaction - base joined to C1 of pentose to form nucleoside
[ base + sugar —> nucleoside + water]
3) 2nd condensation reaction - phosphoric acid joined to C5 of pentose to form nucleotide [ nucleoside + phosphoric acid —> nucleotide + water]
How are dinucleotides formed?
1) via condensation
2) between phosphate group at C5 of pentose sugar of one nucleotide
3) and hydroxyl group at C3 of pentose sugar of adjacent nucleotide
4) forms a phosphodiester bond
How are polynucleotides formed?
- Condensation reaction repeated several million times, 1 molecule of water removed each time
- Covalent bond formed is an ester bond, aka phosphodiester bond —> strong covalent bond confers strength and stability of polynucleotides
- Consists of negatively-charged sugar-phosphate backbone
- 5’ end: phosphate group attached to C5 of pentose sugar
- 3’ end: hydroxyl group on C3 of pentose sugar
- Nucleotides only added to 3’ end of polynucleotide chain (5’ -> 3’ orientation, DNA sequence also read in this direction)
What is the difference between:
- DNA molecule
- DNA strand
- DNA polynucleotide
- DNA molecule: genetic term, typically refers to double stranded DNA, or single stranded DNA or circular or linear forms
- DNA strands: refers to one of the 2 strands of double stranded DNA molecule
- DNA polynucleotide: refer to polymer of deoxyribonucleotides joined by phosphodiester bond
What is complementary base pairing in DNA?
- Bases H-bond to each other as purine-pyrimidine pairs
- A to T (in DNA)/ A to U (in RNA): 2 H-bonds
- G to C: 3 H-bonds
- Ratio of A to T = 1:1, Ratio of G:C = 1:1 and ratio of (A+T) to ratio of (G+C)= 1
- Width between 2 sugar phosphate backbones is constant = 2.0nm
What is a right-handed helix?
2 separate polynucleotide chains are wound around each other to form right-handed double helix
What does anti-parallel mean?
- Two complementary strands are oriented in opposite directions
- One strand runs in 5’ -> 3’ direction
- Another strand runs in 3’ -> 5’ direction
What is the significance of sugar-phosphate backbone and planar bases?
- Negatively-charged sugar-phosphate backbones of DNA on the outside, bases project at right angles inwards
- Relatively hydrophobic nitrogenous bases are “hidden” in interior of helix —> DNA is soluble in water
What are major and minor grooves?
- Between backbone strands run the major and minor grooves, which follow a helical path
- Major: 22A wide and minor: 12A wide
- Edges of the bases more accessible in major groove —> proteins (eg transcription factors) bind to specific sequences of DNA at sides of bases exposed in major groove
What is the dimension of DNA double helix?
- Base pairs are 0.34nm apart
2. One complete turn of DNA double helix has 10 base pairs, spans a distance of 3.4nm
Where is the mRNA manufactured?
- Synthesized along DNA in the nucleus via transcription, passed out into cytoplasm through nuclear pore
- Only 1 strand of DNA is copied (template strand)
- Base sequence of mRNA is complementary to template DNA strand
What are the main functions of mRNA?
- Carries genetic information from nucleus to cytoplasm for protein synthesis
- Provides specific binding sites for a series of complementary aminoacyl-tRNA complexes —> hence codes for specific amino acid sequence in polypeptide synthesis during translation
What are the main structural features of tRNA?
- 74 - 95 nucleotides, smallest of all RNAs
- at least 20 different kinds of tRNA
- H bonding between complementary bases along chain —> strand folds up, forming a 2D clover-shaped structure
- Strand then folds back upon itself to form a compact 3D L-shaped structure
- contains anticodon: sequences of 3 nucleotides complementary to codons on mRNA
- Amino acid attachment site: 5’ end of tRNA always ends with guanine, 3’ always ends with base sequence CCA (amino acid attachment site)
- Aminoacyl-tRNA synthetases are responsible for attaching amino acids to tRNA molecules —> ensures that tRNA having a particular anticodon attaches to only one kind of amino acid
What is the main function of tRNA?
- To carry a specific amino acid to a specific codon on the mRNA so that sequence of information can be maintained
- Recognises the appropriate codons in the mRNA and picking up the appropriate amino acid
What are the main structural features of rRNA?
- Ribosomal proteins formed in cytoplasm enter nucleus via nuclear pores, associate with rRNA to form small and large ribosomal units
- Ribosomal units exit nucleus via nuclear pore, assemble on mRNA to form functional ribosomes
What are the main functions of rRNA?
- Helps to align mRNA in ribosome in such a way that aminoacyl-tRNA complexes can base-pair conveniently
- Provides the structural framework for holding mRNA and tRNA, helps to stabilise temporary union between mRNA and tRNA
