The two strands are held together by hydrogen bonds between the bases, with adenine forming a base pair with thymine, and cytosine forming a base pair with guanine. Base pair describes the relationship between the building blocks on the strands of DNA.
And each of the nucleotides on one side of the strand pairs with a specific nucleotide on the other side of the strand, and this makes up the double helix. So for example, if there's a G on one side of the strand, there will always be a C on the other.
If there's a T on one side of the strand, there will always be an A on the other. How does DNA base pairing work? Mar 6, Explanation: Adenine and Guanine are purines and Cytosine and Thymine are pyrimidines.
Bonded together, these base pairs form the rungs of the DNA ladder or double helix. Related questions Why is the number of chromosomes an organisms has an even number? Why are chromosomes important to heredity? Why is DNA called the blueprint of life? In the s, Friedrich Miescher Figure 1 , a physician by profession, was the first person to isolate phosphate-rich chemicals from white blood cells or leukocytes.
He named these chemicals which would eventually be known as RNA and DNA nuclein because they were isolated from the nuclei of the cells. To see Miescher conduct an experiment step-by-step, click through this review of how he discovered the key role of DNA and proteins in the nucleus.
In , he reported the first demonstration of bacterial transformation , a process in which external DNA is taken up by a cell, thereby changing morphology and physiology. He was working with Streptococcus pneumoniae, the bacterium that causes pneumonia. Griffith worked with two strains, rough R and smooth S.
The R strain is non-pathogenic does not cause disease and is called rough because its outer surface is a cell wall and lacks a capsule; as a result, the cell surface appears uneven under the microscope. The S strain is pathogenic disease-causing and has a capsule outside its cell wall. As a result, it has a smooth appearance under the microscope.
Griffith injected the live R strain into mice and they survived. In another experiment, when he injected mice with the heat-killed S strain, they also survived. In a third set of experiments, a mixture of live R strain and heat-killed S strain were injected into mice, and—to his surprise—the mice died. Upon isolating the live bacteria from the dead mouse, only the S strain of bacteria was recovered.
When this isolated S strain was injected into fresh mice, the mice died. Griffith concluded that something had passed from the heat-killed S strain into the live R strain and transformed it into the pathogenic S strain, and he called this the transforming principle Figure 2. Figure 2. Two strains of S. The R strain is non-pathogenic. The S strain is pathogenic and causes death. When Griffith injected a mouse with the heat-killed S strain and a live R strain, the mouse died.
The S strain was recovered from the dead mouse. Thus, Griffith concluded that something had passed from the heat-killed S strain to the R strain, transforming the R strain into S strain in the process. They isolated the S strain from the dead mice and isolated the proteins and nucleic acids, namely RNA and DNA, as these were possible candidates for the molecule of heredity. They conducted a systematic elimination study. They used enzymes that specifically degraded each component and then used each mixture separately to transform the R strain.
They found that when DNA was degraded, the resulting mixture was no longer able to transform the bacteria, whereas all of the other combinations were able to transform the bacteria. This led them to conclude that DNA was the transforming principle. DNA evidence was used for the first time to solve an immigration case. The story started with a teenage boy returning to London from Ghana to be with his mother. Immigration authorities at the airport were suspicious of him, thinking that he was traveling on a forged passport.
After much persuasion, he was allowed to go live with his mother, but the immigration authorities did not drop the case against him. All types of evidence, including photographs, were provided to the authorities, but deportation proceedings were started nevertheless. Around the same time, Dr. The immigration authorities approached Dr.
Jeffreys for help. Forensic scientists analyze many items, including documents, handwriting, firearms, and biological samples. They analyze the DNA content of hair, semen, saliva, and blood, and compare it with a database of DNA profiles of known criminals. Only mitochondrial DNA is sequenced for forensics.
Forensic scientists are expected to appear at court hearings to present their findings. They are usually employed in crime labs of city and state government agencies.
Geneticists experimenting with DNA techniques also work for scientific and research organizations, pharmaceutical industries, and college and university labs. Experiments conducted by Martha Chase and Alfred Hershey in provided confirmatory evidence that DNA was the genetic material and not proteins. Chase and Hershey were studying a bacteriophage, which is a virus that infects bacteria.
Viruses typically have a simple structure: a protein coat, called the capsid, and a nucleic acid core that contains the genetic material, either DNA or RNA. The bacteriophage infects the host bacterial cell by attaching to its surface, and then it injects its nucleic acids inside the cell.
The phage DNA makes multiple copies of itself using the host machinery, and eventually the host cell bursts, releasing a large number of bacteriophages. Hershey and Chase labeled one batch of phage with radioactive sulfur, 35 S, to label the protein coat.
Another batch of phage were labeled with radioactive phosphorus, 32 P. Because phosphorous is found in DNA, but not protein, the DNA and not the protein would be tagged with radioactive phosphorus. Each batch of phage was allowed to infect the cells separately. After infection, the phage bacterial suspension was put in a blender, which caused the phage coat to be detached from the host cell.
The phage and bacterial suspension was spun down in a centrifuge. The heavier bacterial cells settled down and formed a pellet, whereas the lighter phage particles stayed in the supernatant. In the tube that contained phage labeled with 35 S, the supernatant contained the radioactively labeled phage, whereas no radioactivity was detected in the pellet. In the tube that contained the phage labeled with 32 P, the radioactivity was detected in the pellet that contained the heavier bacterial cells, and no radioactivity was detected in the supernatant.
Hershey and Chase concluded that it was the phage DNA that was injected into the cell and carried information to produce more phage particles, thus providing evidence that DNA was the genetic material and not proteins Figure 3. Figure 3. Only 32 P entered the bacterial cells, indicating that DNA is the genetic material. Around this same time, Austrian biochemist Erwin Chargaff examined the content of DNA in different species and found that the amounts of adenine, thymine, guanine, and cytosine were not found in equal quantities, and that it varied from species to species, but not between individuals of the same species.
This finding proved immensely useful when Watson and Crick were getting ready to propose their DNA double helix model. The experiments by Hershey and Chase helped confirm that DNA was the hereditary material on the basis of the finding that:. Figure 4. The three suggested models of DNA replication. The elucidation of the structure of the double helix provided a hint as to how DNA divides and makes copies of itself.
This model suggests that the two strands of the double helix separate during replication, and each strand serves as a template from which the new complementary strand is copied. What was not clear was how the replication took place. There were three models suggested: conservative, semi-conservative, and dispersive see Figure 4. In conservative replication, the parental DNA remains together, and the newly formed daughter strands are together.
Meselson and Stahl were interested in understanding how DNA replicates. They grew E.
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