Post on 21-Jun-2020
CHROMOSOME STRUCTURE &
DNA ORGANIZATION
Bacterial & Viral Chromosomes
• Viruses
– DNA or RNA
– SS or DS
– Linear or circular
• Bacteria
– Circular DNA
– DS DNA
– 1 chromosome; possibly multiple plasmids
Bacterial & Viral Chromosomes
• Universal problem
– How is the vast amount of genetic material
contained?
Fig. 12-3
Bacterial & Viral Chromosomes
• Viruses
– Genetic material tightly packed into capsid
– Functionally inert until released into host
• Bacteria
– DNA compacted into nucleoid
– Associated with DNA-binding proteins
Clicker question
• A)
• B)
• C)
• D)
Eukaryotic Chromosomes
• Humans
– Longest human chromosome = #1
• Unfolded ~10 cm
– All chromosomes unfolded ~2 m
– Cell nucleus ~5-8 m diameter
Eukaryotic Chromosomes
Fig. 12-11
Chromatin Structure
• Chromatin is a stable, ordered complex of DNA and protein
• Histones = major class of basic proteins in chromatin fibers – Positively charged
– Associate with phosphates in DNA backbone
• Five major types of histones are found in chromatin:
H1, H2A, H2B, H3 and H4
• Histones of different species are similar = conserved
Histones
H2A, H2B, H3, H4:
Form octamers (=2 tetramers)
– Associate with 147 bp of
DNA
H1 subunits associate with linker
DNA between nucleosomes
Fig. 11-11
Nucleosomes
Fig. 11-10
Eukaryotic Chromosomes
– Solenoid formation
• Coil of nucleosomes
– Looped domains
• Coil of solenoids
• Chromatin
– Chromatid formation
• Prophase of mitosis (or meiosis)
Fig. 11-11
Forms of Chromatin
• How does the degree of chromatin folding
affect transcription?
Forms of Chromatin
• Heterochromatin
– Compact regions
– Rich in satellite DNA
– Low in gene content
– Higher degree of folding
– Genetically inactive
• Centromeres & telomeres – repetitive sequences
• Introns
• Most of Barr bodies
Forms of Chromatin
• Euchromatin
– Less condensed
– High gene content (active “structural” genes)
– More diffuse (less folding)
• Satellite DNA
– Highly repetitive non-coding DNA sequences
Genome Organization in
Eukaryotes • Most of the genome does not encode
functional genes
So what is most of the DNA?
Fig. 12-13
Centromeric & Telomeric DNA
• Satellite DNA
– Short sequences repeated many times
– CEN regions all very similar
• 170 bp in tandem arrays up to 1M bp
– Telomere regions
• Short tandem repeats
Fig. 12-16
Middle Repetitive Tandem
Repeats • No known function in genome
– Often used as forensics markers
• Minisatellites
– VNTRs (variable number tandem repeats)
– 15-100bp sequences
– Form tandem repeats 1000-5000bp long
– Within & between genes
• Microsatellites
– Dinucleotides - (CA)n
Transposable Elements
• No known genomic function
• Able to move to different locations within
the genome
• SINEs (short interspersed elements)
– <500bp; present 500,000+ times in genome
– Alu family
• >5% of human genome
• Origin? RNA element dispersed through genome
via reverse transcriptase
Transposable Elements
• No known genomic function
• Able to move to different locations within
the genome
• SINEs (short interspersed elements)
• LINEs (long interspersed elements)
– L1 family
• >5% of human genome
• Retrotransposon
– Portion of L1 codes for reverse transcriptase
– L1 transcribed RT makes DNA complement new L1
integrated into new site on chromosome
Mitochondria & Chloroplasts
• mtDNA & cpDNA
– Circular DS DNA • Few associated proteins
• Small genomes
• Multiple copies
– Code for certain products essential for transcription/translation, cellular respiration & photosynthesis
– Involves unique ribosomes
• Endosymbiont hypothesis
mtDNA
mtDNA
• Introns & gene repetitions
sequences typically absent
• Replication dependent on nuclear
enzymes
• Respiratory proteins
– Quaternary structure derived from
nuclear and mt transcripts
Fig. 12-5
cpDNA
• Contains introns & repetitive sequences
• Larger genome & more copies than in
mitochondria