Chapters 6 and 3 Presented by Dr Laurie M Erickson Chair of the Department of Health Sciences Blitstein Institute of Hebrew Theological College Chicago IL USA 1 61 Genome Size Genome ID: 931172
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Slide1
Lecture 2:
DNA Structure and Chromosome Structure(Chapters 6 and 3)
Presented by Dr. Laurie M. EricksonChair of the Department of Health SciencesBlitstein Institute of Hebrew Theological CollegeChicago, IL, USA.
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Slide26.1 Genome Size
Genome: the genetic complement of a cell or virusOne complete haploid set of chromosomes,All of the genesC-value:
the DNA content of the haploid genome. Measured in basepairskilobase (kb) 10
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basepairs. megabase (Mb) 106 basepairsViral genomes range from 100–1000 kb.The tiny virus MS2 has only four genes and a genome of 4 kbBacterial genomes range 1000–10000 kb.E. coli has a circular DNA molecule of 4600 kb. Eukaryotic genomes range 100–1000 Mb: Fruit fly genome (Drosophila melanogaster) is 180 MbHuman genome is 3000 Mb.
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Slide3The C-value Paradox
In eukaryotes, genome size often differs tremendously, even among closely related speciesGenome size among species of protozoa differ by 5800-fold, among arthropods by 250-fold, fish 350-fold, algae 5000-fold, and angiosperms 1000-fold.C-value paradox: Among eukaryotes, there is NO consistent relationship between the C-value and the metabolic, developmental, or behavioral complexity of the organism.In eukaryotes:
DNA can have special functions, structural or regulatory.NOT coding for the amino acid sequence of proteins
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Slide4The C-value Paradox
Table 6.1, enlarged.
kb
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Slide56.2 Chemical Composition of DNA
DNA is a linear polymer of four deoxyribonucleotidesNucleotidesfive-carbon sugar (ribose or deoxyribose). 5’ and 3’ carbons.phosphoric acidnitrogen-containing base. A, T, G, or C.Nucleoside:
precursor molecule, no phosphate.
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Slide6Adenine (A) and Guanine (G) are Purines
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Slide7Thymine (T) and Cytosine (C) are Pyrimidines
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Slide8Phosphodiester bondsThe 5' phosphate (PO
4) is linked to the 3' hydroxyl (OH)New nucleotides will be added to the 3’ OH end.DNA polymerase: enzyme for DNA synthesis
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DNA Structure: phosphodiester bonds to make a chain.
Slide96.3 Duplex DNA Structure
James Watson and Francis Crick. 1953. Nobel Prize.Two polynucleotide chains twisted around each another.Right-handed helix Antiparallel strands. They run in opposite directions.The two chains are connected in the middle by the bases.The bases are connected by weak hydrogen bonds.Adenine pairs with Thymine.
Guanine pairs with Cytosine.Chargaff’s rules. A=T and G=C.The basepairs are planar, parallel to one another, and perpendicular to the long axis of the double helix.
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Slide10Part B Courtesy of Antony M. Dean,
University of Minesota
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Watson-Crick model of DNA structure
Double helix with major and minor grooves
Diameter of the helix = 20
A
o
Each turn of the helix = 10 bases= 34
A
o
Slide11Hydrogen bonds between
basepairs
.
“complementary”
Slide126.4 DNA Replication
Strand separation: hydrogen bonds break. Helix opens.Template: each DNA strand is the template for the synthesis of a new strandTemplate (parental) strand determines the sequence of bases in the new daughter strand.
Complementary rulesSemi-conservative model: new DNA has one old strand and one new strand.Meselson and Stahl experiment.
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Slide1313
Meselson and Stahl experiment,
1958
Slide14E. coli has circular chromosome.Replication begins at a single site, the
origin of replication.DNA synthesis is bidirectional.Replication fork: the region in which parental strands are separating and new strands are being synthesized.
DNA replication in E. coli.
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Slide15DNA replication in Viruses
Rolling circle replication for viruses with circular DNA molecules. One DNA strand is cut to produce a 3'-OH end. New nucleotides are added to 3’OH end, using uncut strand as template.5’ end is displaced, makes long tail of repeated copies.Complementary DNA strand is made later.
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Slide16DNA Replication in Eukaryotes
Linear DNAVery long molecule. Bidirectional replication
Multiple origins of replicationreduces the total replication time. origins of replication are about 40 kb apartReplication of all chromosomes in eukaryotes usually takes 5-10 hours.
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Slide17DNA replication has Leading and Lagging Strands
Leading strand is synthesized continuously in 5’ – 3’ direction.Lagging strand is made in small fragments = Okazaki fragments. 5’– 3’ direction on antiparallel strandEventually joined to make single strand.
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Slide18RNA is needed to begin DNA Replication
Differences between DNA and RNADNA sugar = deoxyribose RNA sugar = riboseRNA contains the pyrimidine uracil (U) instead of thymine (T)
DNA is double-stranded. RNA is single-stranded.
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Short RNA fragment serves as a
primer
to initiate DNA synthesis.
Made by special RNA polymerase
Removed later
Slide19Summary of DNA Replication
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Slide20Final Steps in Eukaryotic DNA Replication
Removal of the RNA primerBacteria have a special DNA polymerase (Pol I) that removes one ribonucleotide at a time.Eukaryotes have RPA (replication protein A), which removes the entire primer RNA at once. Replacement with a DNA sequence Joining adjacent DNA fragments (Okazaki fragments)
DNA ligase makes the phosphodiester bond.
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Slide213.4 Chromosome Structure
Each eukaryotic chromosome is a single DNA molecule.Coiled up to fit into the nucleus.Visible during the mitosis phase of cell division.Made of chromatin: stable complex of DNA and proteinHistone proteins in chromatin
control chromosome structure during the cell cycle.regulate chromosome functions.Nucleosome: the basic structural unit of chromatinCore particle: DNA is wound around protein core.
Linker DNA between cores: ~55 base pairs
Histone
H1 binds to the core particle and linker DNA21
Slide22Nucleosome Structure
Core particle is an octomer.Two copies of each histone proteins H2A, H2B, H3, and H4 DNA segment of about 145 base pairs
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Slide23Nucleosomes coil to
form larger fibers.
Different parts are more-or-less coiled, depending on their activity.
The chromosome can be seen in the microscope when it is tightly coiled during metaphase.
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DNA is coiled up in the nucleus.
Slide24Chromosome Structure
The spaces between the chromatin form a network of channels Large enough for the enzymes for replication, transcription, and RNA processing.Replication takes place in small discrete regions.Replicated chromosome has two sister chromatids, connected by the centromere.Transcription takes place in a few hundred discrete locations.
Parts of the ChromosomeHeterochromatinEuchromotinCentromereTelomeres
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Slide25Heterochromatin and Euchromatin
HeterochromatinCompact and heavily stained regions of chromatin Satellite DNA: highly repetitive noncoding DNAFew genesEuchromatinUncoiled during regular cell activities.Visible when coiled during mitosis or meiosis.
Many genes
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Euchromatin (purple)
Slide2626
3.5 Centromere and Telomeres
Centromere:
the region that connects the two chromatids.
Specific region of the eukaryotic chromosome.
Central component of the kinetochore machinery during mitosis.
Telomeres:
the two ends of each linear chromosome.
Essential for the stability of the chromosome tips
May be involved in age-related disorders
Special mechanism to restore DNA in telomeres in each cycle of replication.
Telomerase
enzyme
Slide27Next Lecture: Gene ExpressionHow Genes do their Job.
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Slide28The C-value Paradox
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