DNA MODEL

STRUCTURE

• deoxyribonucleic acid

• two strands of polymer wound into a helix

• complementary chains

• nucleotides

• nitrogenous bases (adenine, cytosine,

guanine, thymine)

• sugar (deoxyribose)

• phosphate group

STRUCTURE

• double helix is due to the differences in size of the nitrogenous base • two-ring compound (adenine, guanine) and one-ring compound

(cytosine , thymine)

• helical structure makes DNA equal in distance

• Nitrogenous bases can only fit together if arrange in antiparallel

• twisting is right-handed

• contains major and minor grooves

DNA Conformations

• B – DNA (B form)

• observed in high humidity

• corresponds to the average structure of the

DNA in physiological conditions

• 10 base pairs per turn

• wide major groove and narrow minor groove

• right handed helical

• long and thinner

DNA Conformations

• A – DNA (A form)

• observed in low humidity

• some DNA adopts this structure in protein

complex

• double helix RNA are in A-form

• 11 base pairs per turn

• narrow and deep major groove and broad and

shallow minor groove

• right handed helical

• short and broad

DNA Conformations

• Z – DNA (Z form)

• found in solutions containing high

concentrations of + charged ions (sodium)

• unknown physiological significance but found

in cell’s DNA in small portions

• 12 base pairs per turn

• flattened major groove and extremely narrow

and deep minor groove

• left handed helical

• elongated and slim

DNA Replication • DNA templating

• requires separation of the helix

• recognition of nucleotides by free complementary nucleotides r

• DNA replication fork is asymmetrical • leading strand and a lagging strand

• DNA replication starts in origins of replication • recognized by the protein origin recognition complex

• DNA contains A and Ts

CENTRAL DOGMA OF MOLECULAR BIOLOGY

DNA Replication

DNA Replication

• DNA polymerase – responsible in adding nucleotides

during replication

• helicase – unwinds the DNA so that DNA polymerase can

attach to the template strand and start replication

• DNA ligase – responsible in putting together Okazaki

fragments

Transcription

• DNA strands separated temporarily to produce a

formation that allows the insertion of RNA polymerase

• RNA polymerase adds complementary nucleotides (A - U

and C - G)

Transcription

Transcription

• exons (expressed sequences)

• RNA splicing

• mRNA

Translation • Genetic code • sequence of nucleotides in mRNA is read in consecutive

groups of three

• RNA is a linear polymer of 4 different

nucleotides (A,U,G,C) • if read in groups of three 4 X 4 X 4 = 64 possible

combinations of AA

20 different AA are known

• the genetic code is redundant and some amino acids are

specified by more than one triplet

Translation

Translation