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Chapter 96 -- Part 1: The Search for Heritable Material
Several different strains of the bacterium Diplococcus pneumoniae were used in the experiments.
Some strains cause pneumonia in humans and mice and some are harmless.
The ability ofbacteria to transform harmless cells into virulent ones was discovered by Griffith.
The experiment is known as the transformation experiment.
You can read about the information about the transformation ofbacteria later in the chapter.
You won't be tested on the names of the scientists if you see questions that are based on the classic experiments on the AP exam.
In 1944, the authors published their findings that the transformation factor is in fact DNA.
The research showed that the genetic characteristics of dead and livingbacteria were carried by the same agent.
This provided direct experimental evidence that the genetic material was DNA.
Hershey and Chase carried out experiments that supported the theory that DNA is the genetic material.
The radioactive 32 P and 35 S were used to identify the genes of the viruses.
The radioactive sulfur in the phage always entered the bacterium, while the radioactive phosphorus always entered the cells.
This proved that the viral nucleus, not the viral coat, was the cause of the infections.
The X-ray crystallography analysis of the DNA showed that it was a helix.
Her work was important to Crick.
Maurice Wilkins shared the prize with other people.
She died before the prize was awarded.
A number of important historic experiments were involved in the proof that DNA is the carrier of genetic information.
The double helix structure of DNA was proposed in a one-page paper in the British journal Nature.
Many scientists worked to understand the structure of DNA during the 1940s.
The data that was used to build the model of DNA was derived from other scientists.
The X-ray diffraction analysis of DNA was one of the two major pieces of information they used.
The brilliance of their achievement is unaffected by the fact that much of the components of DNA were known before they began their model building.
Understanding the structure of DNA gives us a foundation to understand how it works.
The 1962 prize for correctly describing the structure of DNA was won byWatson and Crick.
Francis Crick predicted that DNA replicates in a semiconservative fashion.
The heavy nitrogen in the medium they used to cultured thebacteria allowed them to incorporate it into their genes.
After being transferred to a medium containing light nitrogen, thebacteria were allowed to replicate and divide only once.
The final replication was spun in a centrifuge and found to have a similar density between the heavy nitrogen and light nitrogen strains.
The newbacteria contained one heavy strand and one light strand.
The double helix is a twisted ladder with two strands running in opposite directions.
One strand runs from the right side up to the left side down.
The repeating units of nucleotides are in the DNA.
Number 1 to 5 are the carbon atoms in deoxyribose.
There are four nitrogenous bases in DNA.
The purines and pyrimidines of the nitrogenous bases are adenine and guanine.
The nitrogenous bases of opposite chains are coupled to one another by hydrogen bonds.
Through evolutionary history, the base pairing rules of both DNA and RNA are the same.
The nucleus is packed with DNA as needed.
Eukaryotic DNA has a large amount of histones from which it separates only briefly during replication.
The general name for this complex of DNA and histones is chromatin.
The double helix of DNA wraps twice around a core of histones, forming structures called nucleosomes that look like beads on a string.
Adenine, cytosine, guanine, and uracil are the repeating nucleotides that make up the single-stranded helix.
ribose is the 5-carbon sugar.
pyrimidines have a single-ring structure, while purines have a double-ring structure.
The prediction of the making of an exact replica of the DNA molecule by semiconservative replication was proven by Meselson and Stahl.
Each strand of the double helix contains a template for the creation of a new strand composed of T and C with G.
The origin of replication is where the two strands of DNA separate to form bubbles.
There are thousands of bubbles along the DNA molecule.
The process of replicating along the giant DNA molecule is sped up by bubbles.
The Y-shaped region where the new strands of DNA are elongating is at each end of the replication bubble.
All the bubbles come to an end.
The new strands of DNA are antiparallel.
The replication fork moves along the template strand to push the new strand from the 5' to the 3' direction.
The rate of elongation in humans is around 50 nucleotides per 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609- 888-609-
The 3' end of a preexisting chain can only be added to by DNA polymerase.
TheRNA primer is a part of the preexisting chain.
The primer is made by joining togetherRNA nucleotides.
One of your cells can make a copy of its entire genome in a few hours.
The two original strands of DNA are different.
One strand is formed toward the replication fork in a linear fashion despite the fact that it builds both strands in the 5' to 3' direction.
This strand is called the leading strand.
The lagging strand forms in the direction away from the fork in a series of segments.
The fragments are long and will be joined into a single strand by the DNA ligase.
There are other genes that help in the replication of the DNA.
The double helix is twisted at the replication fork.
The parental strands are available as templates.
The two DNA strands are held apart by single-stranded binding proteins.
The tension on the tightly wound helix is lessened by breaking and rejoining the DNA strands.
There is a type of repair called mismatch repair that corrects errors.
The damaged parts of the DNA are excised.
The ends of the chromosomes are lost when the DNA replicates.
To protect against the loss of genes at the ends of the chromosomes, eukaryotes have special nonsense nucleotides at the ends of the chromosomes that repeat thousands of times.
The ends of the chromosomes are called telomeres.
Telomeres are created and maintained.
Every time the DNA is copied, the telomeres get shorter.
The clock may count cell divisions and cause the cell to stop dividing as it ages.
The process of making proteins has been worked out.
The triplet code in DNA is transcribed into a codon sequence inside the nucleus.
A strand of pre-RNA is processed in the nucleus.
The codon sequence leaves the nucleus and is translated into a polypeptide at the ribosome.
There is a sequence of nucleotides in a gene and a sequence of amino acids in aProtein.
The process by which the information in a DNA sequence is copied is called transcription.
There are three types ofRNA that are involved in the synthesis of the molecule.
When a sequence of DNA is expressed, one of two strands of DNA is copied into the messenger RNA.
Structural rRNA is involution in translation.
The ribosome is made up of two small and large subunits.
The ribosome has three binding sites, known as A, P, and E. A ribosome is a synthesis factory.
tRNA is shaped like a cloverleaf and has a binding site for two acids at one end.
The three stages of transcription are initiation, elongation and termination.
The initiation begins when the RNA polymerase recognizes and binding to the DNA at the promoter region.
The promoter tells the promoter where to begin the transcription.
A key area within the promoter, the TATA box, is recognized by a collection of transcription factors.
The completed assembly of transcription factors is called a transcription initiation complex.
The DNA template begins to be transcribed once the promoter is attached.
Elongation of the strand continues as the strand is added to a growing chain.
The base pair rules are C with G and A with U.
A transcription unit is the stretch of DNA that is transcribed into a molecule.
The codons in each unit are used to code for specific acids.
A single gene can be transcribed into a large amount of mRNA in a matter of minutes.
The mechanisms for proofreading are similar to those found in DNA polymerases.
Errors in the DNA sequence can be harmful, but they are usually short-lived.
The final stage is called Termination.
After the AAUAAA is transcribed, longation continues for a short time.
The DNA template is cut free at this point.
The pre-RNA strand is altered before it is sent out of the nucleus to the ribosome.
The details are here.
A modified guanine nucleotide is added to the 5' end.
The cap helps bind the strand to the ribosome.
A poly (A) tail is added to the 3' end.
The tail protects the strand from degradation and facilitates the release of the strand from the nucleus.
Small nuclear ribonucleoproteins, or snRNPs, are used to remove the noncoding regions of the mRNA.
Only exons, which are expressed, can leave the nucleus.
The length of the mRNA that leaves the nucleus is shorter than the original unit.
The primary transcript is the same length as the average length of the human DNA molecule that is transcribed.
An average-size protein of 200 amino acids requires only 1,200 nucleotides of RNA to code.
Out of the original 27,000 nucleotides, 15,800 are noncoding.
Scientists were expecting to find 100,000 genes before the Human Genome Project was done.
They discovered that humans only have about 22,000 genes.
It can be explained when you know the mechanism of alternative RNA splicing.
Depending on which segments of the primary transcript are treated as exons or introns, differentRNA molecules are produced from the same primary transcript.
Cell type control intron-exon choices by binding to regulatory sequences within the primary transcript.
The process by which the codons of an mRNA sequence are changed into an sequenc e is called translation.
A with U and C with G are the base pairs for the ribosome and the cytoplasm.
One end of the molecule has a specific amino acid and the other has a triplet called an anticodon.
Unlike mRNA, which is broken down immediately after being used, tRNA is used repeatedly.
The energy for this process is provided by a molecule called GTP.
The correct tRNA is joined to the correct amino acid by a specific enzyme.
There are only 20 different tRNAs.
Sixty-one of the codons code for amino acids.
One codon, AUG, has two functions; one is a start codon and the other is a code for the amino acid methionine.
Three codons, UAA, UGA, and UAG, are stop codons.
There are anticodons that can recognize two or more different codons.
The rules for the third base of a codon are not as strict as those for the first two bases.
Wobble is a relaxation of base pairs rules.
When the ribosome is attached to the mRNA, initiation begins.
The first codon is always August.
It needs to be positioned correctly in order for it to be transcribed.
A polypeptide chain is formed when the ribosome is brought into being with the help of tRNA.
Several ribosomes in clusters translate one mRNA molecule at the same time.
When a ribosome reaches one of three ends, the strand is terminated.
The ribosome is broken down after the polypeptide is freed.
The complete genetic code is shown.
There are 64 possible combinations of the nitrogenous bases.
The initiation signal for translation is AUG, which codes for methionine.
Three of the codons are stop codons.
There is no ambiguity about the genetic code.
Look at the chart.
There is more than one codon for leucine.
There is no confusion.
This code is universal and unified all life.
It shows that the code originated early in the evolution of life on Earth and that all living things descended from those first ancestral cells.
There are permanent changes in genetic material.
Toxic chemicals and radiation can cause them.
Normal cell functions are disrupted by amutation in body cells.
The genes of a population can be changed by the transmission of gametes.
Natural selection depends on the raw material.
Some parts of the genome are more vulnerable to change than others.
The regions of Cs and Gs are more strongly connected by a triple hydrogen bond than the regions of As and Ts.
The simplest thing to do is a point.
A base-pair substitution is a chemical change in one base pair.
The processing of genetic information is not perfect.
A single point deletion in a single base pair in the gene that codes for hemoglobin leads to the inherited genetic disorder.
Red blood cells can be affected by low oxygen when this point is involved.
A variety of tissues may be deprived of oxygen when red blood cells are abnormal.
There is a chance that a point change could result in a beneficial change for an organisms, because of the wobble in the genetic code.
The silent mutation shown above does not result in a change in the amino acid sequence.
A deletion is the loss of one letter, and an insert is the addition of a letter into the sentence.
The entire reading frame is altered because of the two mutations.
Changes in geno can lead to changes in phenotype.
One of two things can happen as a result of the frameshift.
Either a mutated polypeptide is formed or not.
Knowledge of genetics has been based on work with the simplest biological systems since the early part of the twentieth century.
The understanding of replication, transcription, and translation of DNA was worked out usingbacteria as a model.
The basis for how diseases are treated and how vaccines are developed is their understanding of how viruses andbacteria are transmitted.
A worldwide industry of genetic engineering and recombinant DNA relies onbacteria like Escherichia coli and viruses for research and therapeutic endeavors.
Thomas Hunt Morgan depended on the fruit fly while Gregor Mendel depended on the garden pea.
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