died in 2004 watson and crick described the structure of dna: 1953
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Nucleic Acids are:
DNA (deoxyribonucleic acid): - giant polymers that carry
instructions for making proteins
RNA (ribonucleic acid):- interpret and carry out the instructions
coded in the DNA
Monomers of nucleic acids:nucleotides
A nucleotide consists of a:
Nitrogenous base
Pentose sugar
Phosphate
Backbones of RNA and DNA consist of:
alternating sugars and phosphates
Bases:- are attached to sugars- project from the side
5´
5´ 3´
3´ 3´
3´5´
5´
T
G
C
T
T
T
C
G
A
C
T
G
T
T
G
A
C
G
A
A
G
C
G
T
A
A
C
A
C
A
T A
G C
GC
A T
TA
C G
G
A T
T A
C G
T A
A T
C G
A T
Cartoon ofbase pairing
Cartoon of double helix
DNA is a double helix.
A DNA molecule:consists of two polynucleotide strands each coiled in a right-handed helix (cylindrical spiral)
The two complementary polynucleotide
strands are held together by
hydrogen bonding between the
nitrogenous bases of adjacent nucleotides
Complementary base pairing: particular bases pair only with certain
bases e.g. adenine – thymine makes it possible to copy DNA
molecules faithfully
Chargaff’s rule:A+G = T+C
e.g. If there is 31% adenine in DNA, find the percentage of guanine.
A = T = 31 + 31 = 62%.So C + G = 100 – 62 = 38%. Guanine = 38/2 = 19%
The virus has single-stranded DNA as its genetic material. Explain the evidence from the table which suggests that the DNA is single-stranded. (2)
OrganismPercentage of each base
Adenine Guanine Cytosine ThymineHuman 31.2 18.8 18.8 31.2Cow 27.9 22.1 22.1 27.9Salmon 29.4 20.6 20.6 29.4Rat 28.6 21.4 21.4 28.6 Virus 24.7 24.1 18.5 32.7
amounts of A and T /C and G/complementary bases different; therefore no base-pairing;
RNA DNApentose sugar: ribose
pentose sugar: deoxyribose
bases: adenine, guanine, cytosineuracil
bases: adenineguaninecytosinethymine
RNA DNAratio of:adenine & uracil to cytosine &guanine varies
ratio of:adenine & thymine to cytosine & guanine is one
RNA DNAmanufactures in the nucleus but found throughout the cell
found almost entirely in the nucleus [occurs in mitochondria & chloroplasts]
interpret and carry out the instructions coded in the DNA
carry instructions for making proteins
RNA DNARNA strands are continually made, broken down and reused
DNA is completely protected by the body
RNA DNAmaybe temporary - existing for short periods
permanent
3 basic forms: messenger, transfer and ribosomal RNA
only one basic form