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DNA: the thread of life

Lectured byChompunuch Virunanon

This presentation Partial Fulfillment of the Requirementsfor the 2303107 General Biology teaching, Department of Biology

Chulalongkorn UniversityAcademic Year 2011

Thread of Life" -Mendel's Gene Theory Explained.

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What are genes?

Deoxyribonucleic acid

• Deoxyribonucleic acid or DNA, is the genetic material that provides the blueprint to produce an individual traits.

• Each person’s DNA id distinct and unique. Even identical twins shows minor difference in their DNA sequence.

• Likewise, genetics techniques have many important application in biotechnology, are used in criminal justices, forensic, to provide evidence of guilt or innocent.

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What are SNPs?

DNA: genetic material

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Griffith’s experiment explanation:bacerial transformation experiments indicated the existence of biochemical

genetic material

• In 1920, English microbiologist’ Frederick Griffith, studied a type of bacterium known than Pneumococci and now classified as Streptococcus pneumoniae.

• Some strains of S. pneumoniae secrete polysaccharide capsule, while other strains do not.

S. pneumoniae

Griffith’s experiments that showed the transformation of bacteria by a “transformation principle”

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The Avery, MacLeod and McCarty experiment

Biochemical identification of genetic material

1. Information: The genetic material must contain the information necessary to construct an entire organism

2. Replication: The genetic material must be accurately copied.3. Transmission: After it replicated, the genetic material can be passes

from parent to offspring. It also must pass from cell to cell during the process cell division.

4. Variation: difference in genetic material must account for variation within each species and among different species.

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• Eight years after the famous Avery, MacLeod, andMcCarty experiment was published, two scientistsnamed Alfred Hershey and Martha Chase performed anentirely different type of genetic experiment.

• For their experimental system, they selected anextremely small virus called a bacteriophage (or justphage), which only infects bacterial cells. At that time,scientists knew that when these phage infect a bacterialcell, they somehow “reprogram” the bacterium totransform itself into a factory for producing more phage.

• They also knew that the phage itself does not enter thebacterium during an infection. Rather, a small amount ofmaterial is injected into the bacteria and this materialmust contain all of the information necessary to buildmore phages. Thus, this injected substance is thegenetic material of the phage

T2 bacteriophage

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Left. Electron Micrograph of bacteriophage T4. Right. Model of phage T4. The phage possesses a genome of linear ds DNA contained within an icosahedral head. The tail consists of a hollow core through which the DNA is injected into the host cell. The tail fibers are involved with recognition of specific viral "receptors" on the bacterial cell surface.

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Nucleotides contain a phosphate, a sugar and base

DNA RNA

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Levels of DNA structure to create a chromosome

Where’s DNA locate in cell?

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• An early prediction was that, when DNA isolated from a bacterial or virus is centrifuged, all of the DNA would sediment at the same rate, forming a band in centrifuge tube at the end of the experiment.

• However, in 1963, Jerome Vinograd obtained an unexpected results.

• When he centrifuged circular DNA from Polyoma virus, two band were observed.

The structural characteristics of prokaryotic and viral chromosome

The structural characteristics of prokaryotic and eukaryotic

chromosome

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Prokaryote cell

DNA organization

• DNA Organization In prokaryotes

• DNA Organization In eukaryotes

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The structural characteristics of prokaryotic chromosome

• Bacterial chromosome

Electron micrographs of plasmid DNA (relaxed nonsupercoiled DNA)

Bacterial chromosome

• We learned that the DNA of the bacterium Escherichia coli is located in a central region called the nucleoid.

• If an E. coli cell is lysed gently, the DNA is released in a highly folded state.

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DNA Supercoiling

• 1963 observation that polyoma virus DNA preparations had 3 different “types” of DNA which had uniquely different sedimentation velocities

• 1965, two are circular, one linear– One circular molecule is “underwound”

• Negatively supercoiled (more than 10.4 bp/turn of helix)

• Topoisomers– Created by topoisomerases

• Types I and II

• His investigations of this finding led to an understanding that circular DNA can exist in a relaxed or supercoiled form.

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DNA Organization In Eukaryotes

• DNA protein complex called chromatin– Human chromosomes about 19,000 to 73,000

microns in length, total about 2 meters/cell

– Nucleus about 5-10 microns in diameter

– Condensation about 10,000X

A Microscopists' View of Chromosome Organization

Heterochromatin This is the condensed form of chromatin organization. It is seen as dense patches of chromatin. Sometimes it lines the nuclear membrane, however, it is broken by clear areas at the pores so that transport is allowed. Sometimes, the heterochromatin forms a "cartwheel" pattern. Abundant heterochromatin is seen in resting, or reserve cells such as small lymphocytes (memory cells) waiting for exposure to a foreign antigen. Heterochromatin is considered transcriptionally inactive. See Alberts et al, Molecular Biology of the Cell, Garland Publishing, 1994, pages 352 and 353

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• An important issue in transcription is identifying the right gene

• Different type of gene– Housekeeping genes: encode proteins that

are used all the time

– Other genes are activated only under certain cercumstances

• eg. lactose operon of Escherichia coli

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Euchromatin • Euchromatin is

threadlike, delicate. It is most abundant in active, transcribing cells. Thus, the presence of euchromatin is significant because the regions of DNA to be transcribed or duplicated must uncoil before the genetic code can be read.

Chromatin Structure

• Chromatin proteins subdivided into histones and nonhistones

• Histones– Very high contents of lysine + arginine (20-30%)

– Amino acid sequences very conserved between species

• Histone IV differs by one amino acid between pea and cow

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Chromatin in metaphase

The first level of organization you see is a tangle of 20-30 nm fibers. These are actually coils of the DNA and histones. The figure on the left shows the tangled chromatin fibers in the left panel. Shearing forces can be used to further uncoil and stretch these fibers and the beaded filaments appear. The strands between the beads are segments of double stranded DNA.

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DNA Condensation

• DNA duplex 2 nm in diameter• Nucleosome string about 11 nm in diameter• Nucleosomes form coiled solenoid-shaped fiber

with 6 nucleosomes per spiral– 30 nm fiber

• 30 nm fiber then folded/looped to form 300 nm fiber

• 300 nm fibers coiled/folded to form 700 nm fiber that forms chromatid in meiosis/mitosis

Levels of DNA Condensation

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How DNA and Histones are organized in chromosomes.

How can DNA duplicate itself?

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An overview of DNA replication

• Meselson and Stahl use density of measurement to in vestigate mechanism of DNA replication

Three propose mechanisms for DNA replication

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DNA replication

How gene express?

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The central dogma of molecular biology

• Protein include enzymes that catalyze reaction use to make energy

• Proteins control cellular processes like replication

• Protein provide channels in the membrane for cell to communicate with each other share metabolites

The central dogma

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Molecular gene expression in prokaryote Vs eukaryote

• prokaryote

Molecular gene expression in prokaryote Vs eukaryote

• eukaryote

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How can genetic material transfer from parent to offspring ?

Thread of Life" -Mendel's Gene Theory Explained.

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Mendel’s law1. Law of Segregation (The "First Law")The Law of Segregation states thatwhen any individual producesgametes, the copies of a geneseparate so that each gametereceives only one copy. A gametewill receive one allele or the other.

2. Law of Independent Assortment (The "Second Law")The Law of IndependentAssortment, also known as"Inheritance Law" states that allelesof different genes assortindependently of one anotherduring gamete formation. WhileMendel's experiments with mixingone trait always resulted in a 3:1ratio

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Pedigree

Autosomal dominant or recessive????

Pedigree analysis

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Genetic Technology

The first cloned pet: In 2002 is cat!, called CC (for carbon copy)

Gene cloning is done to achieve one main goal

• Producing large amount of DNA or specific gene

• Expressing the cloned gene to produce the encoded protein

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Gene cloning