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Genome Organization & Protein Synthesis and Processing in Plants. Viral genomes. Viral genomes: ssRNA, dsRNA, ssDNA, dsDNA, linear or ciruclar Viruses with RNA genomes: Almost all plant viruses and some bacterial and animal viruses Genomes are rather small (a few thousand nucleotides)

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viral genomes
Viral genomes
  • Viral genomes: ssRNA, dsRNA, ssDNA, dsDNA, linear or ciruclar
  • Viruses with RNA genomes:
  • Almost all plant viruses and some bacterial and animal viruses
  • Genomes are rather small (a few thousand nucleotides)
  • Viruses with DNA genomes (e.g.lambda = 48,502 bp):
  • Often a circular genome.
  • Replicative form of viral genomes
  • all ssRNA viruses produce dsRNA molecules
  • many linear DNA molecules become circular
  • Molecular weight and contour length:
  • duplex length per nucleotide = 3.4 Å
  • Mol. Weight per base pair = ~ 660
procaryotic genomes
Procaryotic genomes
  • Generally 1 circular chromosome (dsDNA)
  • Usually without introns
  • Relatively high gene density (~2500 genes per mm of E. coli DNA)
  • Contour length of E.coli genome: 1.7 mm
  • Often indigenous plasmids are present
plasmids

-lactamase

Plasmids

ori

Extra chromosomal circular DNAs

  • Found in bacteria, yeast and other fungi
  • Size varies form ~ 3,000 bp to 100,000 bp.
  • Replicate autonomously (origin of replication)
  • May contain resistance genes
  • May be transferred from one bacterium to another
  • May be transferred across kingdoms
  • Multicopy plasmids (~ up to 400 plasmids/per cell)
  • Low copy plasmids (1 –2 copies per cell)
  • Plasmids may be incompatible with each other
  • Are used as vectors that could carry a foreign gene of interest (e.g. insulin)

foreign gene

eukaryotic genome
Eukaryotic genome
  • Moderately repetitive
    • Functional (protein coding, tRNA coding)
    • Unknown function
      • SINEs (short interspersed elements)
        • 200-300 bp
        • 100,000 copies
      • LINEs (long interspersed elements)
        • 1-5 kb
        • 10-10,000 copies
eukaryotic genome6
Eukaryotic genome
  • Highly repetitive
    • Minisatellites
      • Repeats of 14-500 bp
      • 1-5 kb long
      • Scattered throughout genome
    • Microsatellites
      • Repeats up to 13 bp
      • 100s of kb long, 106 copies
      • Around centromere
    • Telomeres
      • Short repeats (6 bp)
      • 250-1,000 at ends of chromosomes
eucaryotic genomes
Eucaryotic genomes
  • Located on several chromosomes
  • Relatively low gene density (50 genes per mm of DNA in humans)
  • Contour length of DNA from a single human cell = 2 meters
  • Approximately 1011 cells = total length 2 x 1011 km
  • Distance between sun and earth (1.5 x 108 km)
  • Human chromosomes vary in length over a 25 fold range
  • Carry organelles genome as well
mitochondrial genome mtdna
Mitochondrial genome (mtDNA)
  • Multiple identical circular chromosomes
  • Size ~15 Kb in animals
  • Size ~ 200 kb to 2,500 kb in plants
  • Over 95% of mitochondrial proteins are encoded in the nuclear genome.
  • Often A+T rich genomes.
  • Mt DNA is replicated before or during mitosis
chloroplast genome cpdna
Chloroplast genome (cpDNA)
  • Multiple circular molecules
  • Size ranges from 120 kb to 160 kb
  • Similar to mtDNA
  • Many chloroplast proteins are encoded in the nucleus (separate signal sequence)
cellular genomes
“Cellular” Genomes

Viruses

Procaryotes

Eucaryotes

Nucleus

Capsid

Plasmids

Viral genome

Bacterial

chromosome

Chromosomes

(Nuclear genome)

Mitochondrial

genome

Chloroplast

genome

Genome: all of an organism’s genes plus intergenicDNA

Intergenic DNA = DNA between genes

estimated genome sizes
Estimated genome sizes

mammals

plants

fungi

bacteria (>100)

mitochondria (~ 100)

viruses (1024)

1e1 1e2 1e3 1e4 1e5 1e6 1e7 1e8 1e9 1e10 1e11 1e12

Size in nucleotides. Number in ( ) = completely sequenced genomes

chromosome organization
Chromosome organization

Eucaryotic chromosome

Centromere

Telomere

Telomere

p-arm

q-arm

  • Centromere:
  • DNA sequence that serve as an attachment for protein during mitosis.
  • In yeast these sequences (~ 130 nts) are very A+T rich.
  • In higher eucaryotes centromers are much longer and contain
  • “satellite DNA”
  • Telomeres:
  • At the end of chromosomes; help stabilize the chromosome
  • In yeast telomeres are ~ 100 bp long (imperfect repeats)
  • Repeats are added by a specific telomerase

5’ – (TxGy)n

3’ – (AxCy)n

x and y = 1 - 4

n = 20 to 100; (1500 in mammals)

gene classification
Gene classification

intergenic

region

non-coding

genes

coding genes

Chromosome

(simplified)

Messenger RNA

Structural RNA

Proteins

transfer

RNA

ribosomal

RNA

other

RNA

Structural proteins

Enzymes

what is a gene
What is a gene ?
  • Definitions
    • Classical definition: Portion of a DNA that determines a single character (phenotype)
    • One gene – one enzyme (Beadle & Tatum 1940): “Every gene encodes the information for one enzyme”
    • One gene – one protein: “One gene contains information for one protein (structural proteins included) one gene – one polypeptide
    • Current definition: A piece of DNA (or in some cases RNA) that contains the primary sequence to produce a functional biological gene product (RNA, protein).
coding region
Coding region

Nucleotides (open reading frame) encoding the amino acid sequence of a protein

The molecular definition of gene includes more than just the coding region

noncoding regions
Noncoding regions
  • Regulatory regions
    • RNA polymerase binding site
    • Transcription factor binding sites
  • Introns
  • Polyadenylation [poly(A)] sites
slide18
Gene

Molecular definition:

Entire nucleic acid sequence necessary for the synthesis of a functional polypeptide (protein chain) or functional RNA

anatomy of a gene
Anatomy of a gene
  • ORF. From start (ATG) to stop (TGA, TAA, TAG)
  • Upstream region with binding site. (e.g. TATA box).
  • Poly-a ‘tail’
  • Splices. Bounded by AG and GT splice signals.
bacterial genes
Bacterial genes
  • Most do not have introns
  • Many are organized in operons: contiguous genes, transcribed as a single polycistronic mRNA, that encode proteins with related functions

Polycistronic mRNA encodes several proteins

bacterial operon
Bacterial operon

What would be the effect of a mutation in the control region (a) compared to a mutation in a structural gene (b)?

eucaryotic genes
Eucaryotic genes

Hemoglobin beta subunit gene

Exon 1

90 bp

Intron A

131 bp

Exon 2

222 bp

Intron B

851 bp

Exon 3

126 bp

Splicing

Introns: intervening sequences within a gene that are not translated

into a protein sequence. Collagen has 50 introns.

Exons: sequences within a gene that encode protein sequences

Splicing: Removal of introns from the mRNA molecule.

regulatory mechanisms
Regulatory mechanisms
  • ‘organize expression of genes’ (function calls)
  • Promoter region (binding site), usually near coding region
  • Binding can block (inhibit) expression
  • Computational challenges
    • Identify binding sites
    • Correlate sequence to expression
eukaryotic genes
Eukaryotic genes
  • Most have introns
  • Produce monocistronic mRNA: only one encoded protein
  • Large
alternative splicing
Alternative splicing
  • Splicing is the removal of introns
  • mRNA from some genes can be spliced into two or more different mRNAs
nonfunctional dna
“Nonfunctional” DNA
  • Higher eukaryotes have a lot of noncoding DNA
  • Some has no known structural or regulatory function (no genes)

80 kb

duplicated genes
Duplicated genes
  • Encode closely related (homologous) proteins
  • Clustered together in genome
  • Formed by duplication of an ancestral gene followed by mutation

Five functional genes and two pseudogenes

pseudogenes
Pseudogenes
  • Nonfunctional copies of genes
  • Formed by duplication of ancestral gene, or reverse transcription (and integration)
  • Not expressed due to mutations that produce a stop codon (nonsense or frameshift) or prevent mRNA processing, or due to lack of regulatory sequences
repetitive dna
Repetitive DNA
  • Moderately repeated DNA
    • Tandemly repeated rRNA, tRNA and histone genes (gene products needed in high amounts)
    • Large duplicated gene families
    • Mobile DNA
  • Simple-sequence DNA
    • Tandemly repeated short sequences
    • Found in centromeres and telomeres (and others)
    • Used in DNA fingerprinting to identify individuals
types of dna repeats
Types of DNA repeats

Perfect repeats vs degenerate repeats

Tandem repeats (e.g. satellite DNA)

5’-CATGTGCTGAAGGCTATGTGCTGCGACG- 3’

3’-GTACACGACTTCCGATACACGACGCTGC- 5’

Inverted repeats (e.g. in transposons)

Loop

5’-CATGTGCTGAAGGCTCAGCACATCGACG- 3’

3’-GTACACGACTTCCGAGTCGTGTAGCTGC- 5’

Stem

  • Form stem-loop structures
  • Palindroms = adjacent inverted repeats
  • (e.g. restriction sites)
  • Form hairpin structures

Hairpin

repetitive sequences
Repetitive sequences

Satellite DNA

Chromosomal DNA

Caesium chloride

density gradient

Repeats in the mouse genome

dna repeats and forensics
DNA repeats and forensics

AluSTXa

  • Gender determination
  • Standard technique: PCR amplification of the amelogenin locus
    • (Males = XY => 103 + 109 bp)
  • AluSTXa Alu insertion on X
  • AluSTYa Alu insertion on Y

M F Suspect

878 bp

556 bp

AluSTYa

X-Y homologous regions

AluSTYa

M F Suspect

X

528 bp

199 bp

Y

Alu sequence

mobile dna
Mobile DNA
  • Move within genomes
  • Most of moderately repeated DNA sequences found throughout higher eukaryotic genomes
    • L1 LINE is ~5% of human DNA (~50,000 copies)
    • Alu is ~5% of human DNA (>500,000 copies)
  • Some encode enzymes that catalyze movement
transposition
Transposition
  • Movement of mobile DNA
  • Involves copying of mobile DNA element and insertion into new site in genome
slide36
Why?
  • Molecular parasite: “selfish DNA”
  • Probably have significant effect on evolution by facilitating gene duplication, which provides the fuel for evolution, and exon shuffling
rna or dna intermediate
RNA or DNA intermediate
  • Transposon moves using DNA intermediate
  • Retrotransposon moves using RNA intermediate
ltr long terminal repeat
LTR (long terminal repeat)
  • Flank viral retrotransposons and retroviruses
  • Contain regulatory sequences

Transcription start site and poly (A) site

lines and sines
LINES and SINES
  • Non-viral retro-transposons
    • RNA intermediate
    • Lack LTR
  • LINES (long interspersed elements)
    • ~6000 to 7000 base pairs
    • L1 LINE (~5% of human DNA)
    • Encode enzymes that catalyze movement
  • SINES (short interspersed elements)
    • ~300 base pairs
    • Alu (~5% of human DNA)
proteins
Proteins
  • Most protein sequences (today) are inferred
  • What’s wrong with this?
  • Proteins (and nucleic acids) are modified
  • ‘mature’ Rna
  • Computational challenges
    • Identify (possible) aspects of molecular life cycle
    • Identify protein-protein and protein-nucleic acid interactions
genetic variation
Genetic variation
  • Variable number tandem repeats (minisatellites). 10-100 bp. Forensic applications.
  • Short tandem repeat polymorphisms (microsatellites). 2-5 bp, 10-30 consecutive copies.
  • Single nucleotide polymorphisms
single nucleotide polymorphisms
Single nucleotide polymorphisms
  • 1/2000 bp.
  • Types
    • Silent
    • Truncating
    • Shifting
  • Significance: much of individual variation.
  • Challenge: correlation to disease
yeast genome
Yeast genome
  • 4.6 x 106 bp. One chromosome. Published 1997.
  • 4,285 protein-coding genes
  • 122 structural RNA genes
  • Repeats. Regulatory elements. Transposons.
  • Lateral transfers.