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Module: BIOM - 1006

Lecturer: Dr Sherwood

Date: 1/11/16

DNA, RNA and Chromosomes
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The DNA double helix is composed of sugar-phosphate backbones with bases attached
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Monophosphate, diphosphate and
triphosphate are common
 This is essentially how many phosphates are bonded to the CH2 group
 The phosphate makes the nucleotide negatively charged

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Nucleotides refer to the structure that is a nitrogenous base, a five-carbon sugar and one or more
phosphate groups bonded together
 Carbon-1 has the base attached
 The bases are bonded to the sugar via N-glycosidic bonds
 Carbon-2(RNA),3 have OH groups
 Carbon-5 has the phosphate attached

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The nitrogenous bases can be separated into 2 classes based on the number of rings they contain:
 Pyrimidines
 These have one nitrogen-containing ring
 They include C, T and U
 U lacks the CH3 group that T has
 Purines
 These have two nitrogen-containing rings
 They include A and G

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The base and sugar without the phosphate are known as nucleosides, whereas with the phosphate
they are nucleotides, the nucleosides are as follows:
 A = adenosine
 G = guanosine
 C = cytidine
 U = uridine
 T = thymidine

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Nucleotides are bonded together with phosphodiester bonds between the 5’ and 3’ carbon groups
 These form nucleic acids
 The linear sequence of nucleotides is often abbreviated to one letter:
 E
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ATGCATTGCA
 The 5’ is always at the left and
the 3’ is always at the right
 The following is an image of a 2-nucleotide
sequence with the phosphodiester bonds shown:

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Two strands of DNA are held together by hydrogen
bonding between the bases:
 A and T have two hydrogen bonds
 C and G have three hydrogen bonds

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The two strands of DNA run antiparallel to each other
 The 5’ of one strand is opposite the 3’ end of
the other strand

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The two strands of DNA wrap around each other to form a right-handed helix with 10 bases per tern
 They have alternating major and minor grooves (see below) that are opposite across from each
other
 The major grooves are when the phosphate groups are far apart
 The minor grooves are when the phosphate groups are closer together

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RNA is single stranded compared to DNA’s double strand
 It does however, form a double stranded structure with itself
(see right) with complementary bonding similar to that of DNA

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Genes are made from DNA and so gene expression can be observed, this
is shown as:
 GENE A -> RNA A -> PROTEIN A
 GENE B -> RNA B -> PROTEIN B etc
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2x109 nucleotides
 This is packaged into 23/24 different chromosomes
 Females have 2 X chromosomes and so have 23 different chromosomes whereas
men have Y chromosomes and so 24 different chromosomes

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A karyotype is the pictorial representation of the chromosomal compliment of an individual organism or
cell
 For a human male – 46XY
 For a human female – 46XX

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The sex chromosomes, X and Y are named differently to the other 22 which are known as autosomes

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From each parent, you inherit:
 One set of chromosomes 1-22
 From mother, an X chromosome
 From father, an X or Y chromosome

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Diseases can be caused by having extra chromosomes (>46)
 Down syndrome is caused by having an extra chromosome 21
 Known as trisomy 21
 Edwards syndrome is caused by having an extra chromosome 18
 Known as trisomy 18



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