Grade 12 Genetics

8/13/2019 Grade 12 Genetics

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Discovered that the nuclei of cells contain large quantities of a substance that does not act likeprotein

He called the substance nuclein because it was found in the nucleus of the cell. Now thesubstance is called deoxyribonucleic acid.

Friedrich Meischer

Discovered that mice died after being injected with a mixture of heat killed S-strain bacteria andliving R-strain bacteria.

He concluded that the S-strain passed its deadly properties to the live non-pathogenic R-strain.Called it the transforming principle because the S-strain must have transformed something in theR-strain to make them deadly.

Avery and his colleagues prepared identical extracts of the heat killed S-strain by growing the cells

in liquid culture, isolating the bacteria, disrupting the cell membranes and collecting the cellcontent. One enzyme was added at a time. One enzyme was added to destroy protein, the secondwas added to destroy RNA and the third one to destroy DNA.

Each enzyme-treated extract was mixed with the live R-strain cells. The only extract that did notcause any transformation in the R-strain was the third enzyme treated extract. The conclusion wasthat DNA was what caused the transformation.

Frederick Griffith

Experiment used bacteriophages, which are viruses that infect bacteria. They have an innernucleic acid core and an outer protein core, called a capsid.

Aimed to determine which part of the virus-the DNA or the protein-enters the bacterial cell andproduces more viruses

Since proteins contain sulphur but DNA does not, they introduced a radioactive source of sulfurinto the protein of the virus. The DNA was labeled with phosphorus since, protein does notcontain phosphorus

Experiment One: A virus with radioactively labeled DNA was allowed to infect E.coli bacteria. Thebacteria was then put in a blender to take the bacteriophage ghosts off the surface. The materialwas centrifuged to separate the infected bacterial cells (formed a pellet at the bottom) and theliquid medium (contains the bacteriophage ghost). The radioactive DNA was in the bacteria not inthe liquid.

Experiment Two: A virus with radioactively labeled protein was allowed to infect E.coli bacteria.The bacteria was then put in a blender to take the bacteriophage ghosts off the surface. Thematerial was centrifuged to separate the infected bacterial cells (formed a pellet at the bottom)and the liquid medium (contains the bacteriophage ghost). The radioactive protein was in theliquid not in the bacteria.

In conclusion was that viral DNA was transferred to the bacterial cells and that viral DNA held thegenetic information needed for the viruses to reproduce.

Hershey and Chase

5.1: Genetic ScientistsNovember-15-135:06 PM

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Isolated two types of nucleic acid: RNA (five carbon sugar) and DNA (five carbon sugar without oneoxygen)

Proposed that RNA and DNA are made up of different units called nucleotides. Each nucleotidewould be composed of one of four nitrogen bases, a sugar molecule and a phosphate group.

Phoebus Levene

The purine bases are adenine and guanine. They have two fused rings in therechemical structures.

The pyrimidine bases are cytosine and thymine. They have a single ring in theirchemical structures. In RNA it is uracil instead of thymine.

There are four types of nitrogen-bases all linked together by covalent bonds. They arecategorized into two different forms: purines and pyrimidines.

Showed that there is variation in the composition of nucleotides among different species.Demonstrated that all DNA maintains certain properties, even though the composition varies. Heobserved that the nucleotides were present in characteristic proportions.

Chargaff's rule; in DNA, the percent composition of adenine is the same as thymine, and thepercent composition of cytosine is the same as guanine.

Erwin Chargaff

Discovered that may proteins have a helix-shaped structureLinus Pauling

Determined that DNA has a definite helix-shape. The structure has two regularly repeatingpatterns-one recurring at intervals of 0.34nm, and the other recurring at intervals of 3.4nm.

Concluded that the nitrogenous bases were located on the inside of the helix structure, and thesugar-phosphate backbone was located on the outside, facing toward the watery nucleus of thecell

Rosalind Franklin

Deduced that DNA has a twisted, ladder-like structure, called a double helix.Watson and Crick

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Deduced that DNA has a twisted, ladder-like structure, called a double helix.Watson and Crick

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There are two polynucleotide strands twist around each other to form the double helix. The strands are comprisedof alternating phosphate groups and sugars. The bases are attached to the sugar and protrude inwards. There is aconstant total distance between backbones

The purine molecule is always paired with the pyrimidine molecule. A-T and C-G. This is called complementary basepairing

Hydrogen bonds link each complementarybase pair. A and T share two and C and Gshare three. This makes C and G slightlystronger.

The two strands of DNA are antiparallel. At the end of a DNA molecule, the 5' end of one strand of DNA lies acrossfrom the 3' end of the complementary strand. The 5' and the 3' come from the numbering of the carbons on the

deoxyribose sugar. The phosphate group is on the 5' carbon and the OH group is on the 3' carbon.

The genome is the complete geneticmakeup of an organism; an organismstotal DNA sequenceThe functional units of DNA are genesbecause their sequences code for theproduction of specific proteins or RNAFor most prokaryotes genetic material is

in the form of a circular, double stranded

DNA molecule. Bacterial chromosome istightly packed into the nucleoid.Specialized proteins that bind to thebacterial DNA help fold sections of thechromosome into loop-like structuresDNA supercoiling is when the formation ofadditional coils in the structure of DNAdue to twisting forces on the molecule.In bacteria, the amount of supercoiling iscontrolled by two enzymes:topoisomerase I and topoisomerase II.Antibacterial drugs stop the functionality

of these enzymes.Some prokaryotes have one or two small,circular or linear DNA molecules calledplasmids. They are not part of the nucleoidand carry non-essential genes.Most prokaryotes only carry one copy oftheir genes, so they are haploid organisms.Their genomes contain very little non-essential DNA.

Regulatory sequences are sections of DNA sequences that determine when certain genes and the associated cell

functions are activated.For eukaryotic cells, the total amount of DNA is much greater than prokaryotic cells. DNA is also contained withthe nucleus.Eukaryotes have to fold much more than prokaryotes. There are different levels of organization that takes placewith each chromosome.

5.1: DNA StructureNovember-16-131:29 PM

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A) DNA associates with proteins, histones, to form a repeated series of structures called nucleosomes. Eachnucleosome is composed of double-stranded DNA that is wrapped around a group of eight histone proteins.These nucleosome structures are connected by regions called linker DNA.B) Coiling of the nucleosomes with the help of H1 histone proteins, into which is often called a 30nm fibre. DNAis compacted 50 timesC) Formation of radial loop domains of the 30nm fibre. They are anchored to a scaffold of proteins in thenucleus.Genetic material appears as a mass of long, intertwined strands known as chromatin. The 30nm fibre as loopeddomains, called euchromatin (can undergo further compacting) and heterochromatin.

Most eukaryotes are diploid which means theycontain two copies of each gene. However somemay be haploid and can only carry one. Some maycarry three or more copies of their genes.There is no correlation between an organismscomplexity and genome size or number of protein-coding genes

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DNA replication is the process of producing two identical DNA molecules from an original, parent DNA molecule.Due to complementary base pairing each strand can serve as a template for the production for a newcomplementary strandThere three main models for replication of DNA:Conservative model (two new daughter strand from the parent templates, with two new strands joining to create anew double helix)Semi-conservative model (each new DNA molecule would contain one strand of the new complementary DNAmolecule and one new parent strand. Each new DNA molecule would conserve half of the strand of the originalmolecule)The dispersive model (proposed that the parental DNA fragments were broken into fragments and that bothstrands of DNA in each of the daughter molecules were made up of an assortment of parental and new DNA)

Matthew Meselson and Franklin Stahl reasoned thatthe proposed models for DNA replication could betested if they could distinguish between the originalparent strand and the new daughter strands.They used two different isotopes of nitrogen to labelthe DNA in the cell: the common form of nitrogen,14N, and a rarer form of heavy nitrogen, 15N.The light and heavy forms of nitrogen were usedbecause it was easy to separate the different DNAstrands based on how much of each isotope is presentin a newly synthesized DNA molecule.

Bacterial cells were grown for several generationin medium containing a heavy isotope ofnitrogen. Therefore they contained heavy

nitrogen.Cell were transferred to a new mediumcontaining normal, light nitrogen. At varioustimes after, samples were collected.The DNA was extracted from the cells and put ina centrifuge so that a concentration gradientcould be observed. Heavier sediments formrings toward the bottom.After one generation of growth in 14N medium,the bacteria yielded a single band of DNA with adensity between that of 14N-DNA and 15N-DNAindicating that one strand of each contained 15NAfter two generations of growth in 14N medium,

the bacteria yielded two bands. One containedno 15N and the other contained 14N and 15N.

5.2 DNA ReplicationNovember-16-136:53 PM

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There are three basic phases in replication that rely onthe structural feature of DNA and a number ofspecialized proteins.1) In the initiation phase, a portion of the DNA helix isunwound to expose the bases for new base pairing.2) In the elongation phase, two new strands of DNA areassembled using the parent DNA as a template. The newDNA molecules - each composed of one strand of parentDNA and one strand of daughter DNA - re-form into

double helices.3) In the termination phase, the replication process iscompleted and the two new DNA molecules separatefrom each other.All these events take place simultaneously.Replication starts at a specific nucleotide sequencecalled the origin of replication. Several proteins bind tothe DNA and unwind the double helix.

The helicase enzyme cleave the hydrogen bonds that link the complementary base pairs together. Single-strand-bidingproteins help to stabilize the newly unwound strands (they have a tendency to get back together). The topoisomerase IIenzyme helps relieve the strain on the double helix sections ahead of the replication forks.Initiation creates an unwound, oval shape called the replication bubble, with two Y-shaped regions at each unwound area. TheY-area is called the replication fork.DNA polymerase III is the enzyme that catalyzes the addition of new nucleotides, to create a new strand of DNA that iscomplementary to a parental strand. The enzyme only attaches new nucleotides to the 3' OH end of a pre -existing chain ofnucleotides. It can only synthesis a new strand from a parent strand in the 5' to 3' direction.When double-stranded DNA is separated the strands do not have free 3'hydroxyl end for DNA polymerase to begin at.Therefore, new DNA requires both strands to be started with short fragments of nucleotide sequences, complementary to thetemplates.For once strand, the leading strand, this only need to happen once. DNA polymerase will keep adding new nucleotides to the3' end as i t moves in the same direction as the replication fork.Synthesis of the other end, lagging strand, requires the enzyme to move in the opposite direction. This needs to occur in sho rtfragments and in a discontinuous manner. These short segments are called the Okazaki fragments.

RNA primers are needed to start the replication. RNA primers are synthesized by an enzyme called primase. DNA polymeraseextends the strand by adding new nucleotides to the 3'end of the primer. Once each primer is in place, a new DNA fragment isgenerated at the end of each prmer.The result is short segments containing uracil instead of thymine. Another enzyme, DNA primase I, is needed to replace theuracil. Then the two strands are joined together by DNA ligase.

As soon as the newly formed strands are complete, they rewind automatically.The DNA complex at each replication fork that carries out replication is referred to as the replication machine.The termination phase occurs upon the new DNA strands completion and the two new DNA molecules separate

from each other.One type of error that can take place is a mismatching between the new nucleotide and the template.Another type of problem that can occur is due to strand slippage, which causes either additions or omissions ofnucleotides. This type of error can result from either the newly synthesizing strand looping out, allowing addition ofan extra nucleotide or the looping out of the template strand, resulting in a nucleotide not being added where itshould.Enzymes DNA polymerase I and II proofread each nucleotide and recognize whether or not the correct nucleotide

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has been added. If it detects an error it wil l halt production and fix it.Another mechanism for correcting errors is called mismatch repair. Mismatching pairs cause deformities in themolecule. Special enzymes bind to these parts of the daughter cell and repair them.Telomere is a repetitive section of DNA, near the end of each chromosome; this helps to protect from loss ofinformation during replication of the linear DNA in eukaryotic cells

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Grade 12 Genetics