Upload
burian
View
37
Download
0
Tags:
Embed Size (px)
DESCRIPTION
Lecture 2 Properties of DNA. Reference: Chapter 28: Biochemistry by Voet and Voet. Lecture 2: Outline. C-value paradox Gene size, genome size Topology of DNA Denaturation and renaturation of DNA. gene sizes. Largest known mammalian gene is DMD gene 2.5 Mbp (0.1% of the genome) - PowerPoint PPT Presentation
Citation preview
Lecture 2 Properties of DNA
Reference:Chapter 28: Biochemistry by Voet and Voet
C-value paradoxC-value paradox
Gene size, genome sizeGene size, genome size
Topology of DNA Topology of DNA
Denaturation and renaturation of DNADenaturation and renaturation of DNA
Lecture 2: Outline
gene sizes Largest known mammalian gene is
DMD gene 2.5 Mbp (0.1% of the genome)
Causes Duchenne’s muscular dystrophy (DMD)
characterized by rapid progression of muscle degeneration which occurs early in life.
‘scoliosis’
DMDMDDDMD gene maps to
chromosome Xp21DMD affects mainly malesMutations in DMD gene lead to non functional dystrophin protein
genome sizes
organism Number of base pairs (kb)
virusesLambda bacteriophage ( λ) 48.6
bacteriaEschericia coli 4,640
eukaryotesYeast 13,500Drosophila 165,000Human 3.3 x 106
Does size matter? C-value Does size matter? C-value paradoxparadox
mountain grasshopper Podisma pedestrisGenome size: 18 Gbp
protozoan Amoeba dubiaGenome size: 670Gbp
Boa constrictorGenome size: 2.1 Gbp
Homo sapiens sapiensGenome size: 3.2 Gbp
C value: DNA content of a haploid cellC value: DNA content of a haploid cell
CComparatiomparative genome ve genome sizessizes
Why is there a discrepancy between genome size and genetic complexity?
C-value paradox
Due to the presence of repetitive (junk?) DNA
Repetitive DNA families constitute nearly one-half of genome (~45%)
Protein domains contribute to organism complexity
Topology of DNA
DNA supercoiling: coiling of a coil
Important feature in all chromosomes
Supercoiled DNA moves faster than relaxed DNA
Allows packing / unpacking of DNA
negatively supercoiled (right handed)•Results from under or unwinding•Important in DNA packing/unpacking e.g during replication/transcription
positively supercoiled (left handed)•Results from overwinding•Also packs DNA but difficult to unwind
Supercoiling topologySupercoiling topology
No supercoiling (left) to tightly supercoiled (right)
Analogy = phone cord
Supercoiling takes 2 forms Supercoiling takes 2 forms toroidal (DNA around histones) or toroidal (DNA around histones) or
interwound (bacterial chromosomes)interwound (bacterial chromosomes)
Supercoiled DNA needs to be Supercoiled DNA needs to be unwound /wound when necessary unwound /wound when necessary Unwound during transcription/replicationWound before cell division
Supercoils can be relaxed by Single strand nicks (topoisomerase I) Double strand breaks (toposiomerase II)
Topoisomerase I
Cause temporary single strand breaks in DNA Allows free rotation of helixActs by breaking phosphodiester linkage via tyrosine active site
Topoisomerase II (DNA gyrase)
ATP hydrolysis mediated double strand breakInhibitors are effective antibiotics and cancer chemotherapy agents
supercoiled
Relaxed circle
Full length linear
Why does a plasmid (circular DNA) that has never been cut give more than one band on a gel?
EBr
Denaturation and Denaturation and renaturation of DNArenaturation of DNA
Applications in polymerase chain reaction (PCR)
http://www.dnai.org/b/index.html
Denaturation of DNA
Also called melting
Occurs abruptly at certain temperatures
Tm – temp at which half the helical structure is lost
DNA melting curve
Tm varies according to the GC content
High GC content - high Tm
GC rich regions tend to be gene rich
Renaturation of DNA
Also called annealing
Occurs ~ 25oC below Tm
Property used in PCR and hybridisation techniques