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Transcript of DNA
A Fortuitous Accident!?!
Like many stories in science, the discovery of the molecular nature of the gene began with an investigator who was actually looking for something else. In 1928, British scientist Frederick Griffith made such a discovery while experimenting with bacteria and mice.
Griffith was studying pneumonia-causing strains of bacteria when he made his discovery. He had two different strains of bacteria that he was injecting into mice. One would cause pneumonia and was round-shaped, while the other was harmless but rough-shaped. He would inject the mice with each type of strain and await the results.
When Griffith injected the mice with the disease-causing bacteria, the mice developed pneumonia and died. When he injected the mice with the harmless bacteria, the mice did not get sick at all. Griffith wondered if the disease-causing bacteria produced a poison. To find out, he heated some of the harmful bacteria to kill them. He then injected the dead bacteria into more mice. The mice survived, suggesting that the cause was not a chemical poison produced by the bacteria.
Griffith was not finished with his mice! Next, he mixed live, harmless bacteria with the dead, harmful bacteria in a separate culture and injected the mixture into more mice. To Griffith's surprise, most of the mice he injected with the mixture became sick and died! When he examined the lungs of the dead mice, he found the harmful bacteria present in the lungs. Somehow the heat-killed bacteria had passed their disease-causing ability to the harmless strain. Griffith called this process transformation because one strain had apparently been changed permanently into another type of strain.
Griffith hypothesized that when the two different strains of bacteria were mixed, some factor was transferred from the dead cells into the live cells. That factor must contain information that could change harmless bacteria into disease-causing ones. Furthermore, he hypothesized that since the ability to cause disease was inherited by the transformed bacteria's offspring, the transforming factor might be a gene.
Avery & DNA
In 1944, a group of scientists led by Canadian biologist Oswald Avery decided to repeat Griffith's experiments. They did so to determine which molecule in the heat-killed bacteria was responsible for the transformation. In other words, they were looking for the molecule that might also be the gene. They accomplished their task with the use of enzymes. They would add a specific enzyme that would destroy specific molecules (e.g. an enzyme that only destroys proteins). This method was continued until they had eliminated all possible suspects except one: DNA.
Two American scientists, Alfred Hershey and Martha Chase, experimented with viruses and bacteria in 1952. They were using bacteriophages, viruses that infect bacteria. Bacteriophages are composed of a DNA or RNA core and a protein coat. When a bacteriophage enters a bacterium, the virus attaches to the surface of the cell and injects its genetic information into it. The viral genes act to produce many new bacteriophages, and they gradually destroy the bacterium.
Hershey and Chase reasoned that if they could determine which part of the virus (protein or DNA) entered the infected cell, they would learn whether genes were made of protein or DNA. To do this, they grew viruses in cultures containing radioactive isotopes of phosphorous-32 and sulfur-35. This was clever because proteins contain almost no phosphorous and DNA contains no sulfur. The results of this experiment were conclusive and science had taken another big step in the study of genetics.
Components & Structure of DNA
As you know, the field of science never rests, and scientists were not content with merely identifying the molecule responsible for genes. With the culprit identified, scientists next wanted to know its composition and structure. They knew this molecule had to be special because it was able to carry information from one generation to the next, it was able to use that information by determining the inheritable characteristics, and it had to be easily copied.
DNA is made up of four different types of nucleotides: guanine (G), cytosine (C), adenine (A), and thymine (T). Erwin Chargaff, an American biochemist, had discovered that the percentages of the four nucleotides were almost equal in any sample of DNA. The observation that A=T and C=G, became known as Chargaff's rules.
The two strands of the double helix are held together with hydrogen bonds between the nitrogenous bases. These bonds are just strong enough to keep the two strands connected, but can only form between specific base pairs. Adenine is always paired with Thymine, and Guanine is always paired with Adenine. This principle, which builds upon Chargaff's rules, is called base pairing.
DNA & Chromosomes
Both prokaryotic and eukaryotic cells contain DNA. However, DNA in prokaryotes is floating freely in the cytoplasm of the cell. In eukaryotes, the DNA is stored in the nucleus in the form of a number of chromosomes. Remember, different organisms will contain a different number of chromosomes in each diploid cell. For example, human diploid cells contain 46 chromosomes, while diploid cells from a Sequoia tree contain only 22.
DNA molecules are surprisingly long. For example, the average E. coli bacterium contains 4,639,221 base pairs. Remember, bacteria are considered simple life forms in comparison to eukaryotes. The average length of a strand of DNA from E. coli is 1.6mm, which doesn't seem very long until you compare it to the length of the bacterium in which it is housed. To fit 1.6mm of DNA into an organism as small as a bacterium, the DNA would need to be folded into a space only one one-thousand of its length. This would be like trying to put over 900 feet of rope into your backpack!
The DNA in eukaryotic cells is packed even more tightly. A human cell contains almost 1000 times as many base pairs as a bacterium. The nucleus of a human cell contains more than 1 meter of DNA!! Supposedly, if you were to take all of the DNA molecules out of a human, and stretch them all out in one great length, it would reach from the Sun to Pluto and still have enough to do the trip two more times!!! How on Earth is so much DNA folded into tiny chromosomes? The answer lies in the composition of eukaryotic chromosomes.
Eukaryotic chromosomes contain both DNA and protein, tightly packed together to form a substance called chromatin. Chromatin consists of DNA that is tightly coiled around proteins called histones. Together, they DNA and histone molecules form a beadlike structure called a nucleosome. Nucleosomes pack with one another to form a thick fiber, which is shortened by a system of loops and coils.
The double helix structure of DNA explains how DNA could be copied, or replicated. Each strand of the DNA double helix has all the information needed to reconstruct the other half by the mechanism of base pairing. Because the each strand can be used to make the other strand, the strands are said to be complementary.
Before a cell divides, it duplicates its DNA in a copying process called replication. This process ensures that each resulting cell will have a complete set of DNA molecules. During DNA replication, the DNA molecule separates into two strands, and then produces two new complementary strands following the rules of base pairing. Each strand of the double helix of DNA serves as a template, or model, for the new strand.
DNA replication is carried out by a series of enzymes. These enzymes "unzip" a molecule of DNA. The unzipping occurs when the hydrogen bonds between the base pairs are broken and the two strands of the molecule unwind. Each strand serves as a template for the attachment of complementary bases.
DNA replication involve a host of enzymes and regulatory molecules. Enzymes are highly specific (meaning they do specific things), and for this reason, they are often named for the reactions they catalyze. The principal enzyme involved in DNA replication is called DNA polymerase because it joins individual nucleotides to produce a DNA molecule, which is, of course a polymer. DNA polymerase also "proof reads" each new DNA strand, helping to ensure a perfect copy.
DNA vs RNA
RNA, like DNA, consists of a long chain of nucleotides. Remember, nucleotides are monomers and are made up of a 5-carbon sugar, a phosphate group, and a nitrogenous base. There are three main differences between RNA and DNA:
1. The sugar in RNA is a ribose instead of a deoxyribose.
2. RNA is generally single-stranded.
3. RNA contains the base uracil instead of thymine.
Types of RNA
RNA molecules have many functions, but in the majority of cells, the RNA molecules are involved in protein synthesis. Remember, proteins are made up of monomers called amino acids, and have the most varied functions of all the biomolecules. There are 20 different amino acids, and these 20 can be combined in many different configurations to create different types of proteins. The assembly of amino acids into proteins is controlled by RNA.
Types of RNA
There are 3 main types of RNA:
messenger RNA - mRNA
The RNA molecules that carry copies of the instructions for building proteins.
ribosomal RNA - rRNA
The RNA molecules that, along with proteins, makes up the organelles known as ribosomes.
transfer RNA - tRNA
The RNA molecule that, during protein construction, transfers each amino acid to the ribosome as it is specified by the coded message in mRNA.
Types of RNA
RNA molecules are produced by copying part of the nucleotide sequence of DNA into a complementary sequence in RNA, a process called transcription. Transcription requires an enzyme called RNA polymerase that is similar to DNA polymerase. During transcription, RNA polymerase binds to DNA and separates the DNA strands. RNA polymerase then uses one strand of DNA as a template from which nucleotides are assembled into a strand of RNA.
RNA polymerase knows "where" to bind to the DNA molecule and start transcription and "where" to stop making the copy. This is because RNA polymerase will not bind to DNA just anywhere. The enzyme will bind only to regions of DNA known as promoters, which have specific base sequences. The promoters are basically signals that tell the enzyme when and where to start, as well as when and where to stop.
The Genetic Code
Proteins are made by joining amino acids (monomers) into long chains called polypeptides (polymers). Each polypeptide contains a combination of any or all of the 20 different amino acids. The properties of proteins are determined by the order in which different amino acids are joined together to produce polypeptides. In a way, protein are like words... one different letter/amino acid can make an entirely different word/protein.
The Genetic Code
The "language" of mRNA instructions is called the genetic code. Recall that RNA contains 4 different bases: A, U, C, G. The code is written in a language that only has 4 letters. The genetic code is read three letters at a time, so that each "word" of the coded message is three bases long. Each three letter "word" is known as a codon. A codon consists of three consecutive nucleotides that specify a single amino acid that is to be added to the polypeptide. Consider the following RNA sequence:
Reading Genetic Code
UCG CAC GGU
Now, you try it:
The sequence of nucleotide bases in an mRNA molecule serves as instructions for the order in which amino acids should be joined together to produce a polypeptide. However, anyone who has ever tried to assemble a complex device knows that the instructions don't do the work for you! The instructions need something to read the instructions and perform the assembly. Do you remember which cellular organelle is responsible for assembling proteins?
The decoding of an mRNA message into a polypeptide chain (protein) is known as translation. Translation takes place on ribosomes. During translation, the cell uses information from messenger RNA to produce proteins. Translation begins when an mRNA molecule in the cytoplasm attaches to a ribosome. As each codon of the mRNA molecule moves through the ribosome, the proper amino acid is brought into the ribosome by tRNA. In the ribosome, the amino acid is transferred to the growing polypeptide chain.
Now and then cells make mistakes in copying their own DNA, inserting an incorrect base or even skipping a base as the new strand is put together. These mistakes are called mutations, from a Latin word meaning "to change". Mutations are changes in genetic material.
Mutations that produce changes in a single gene are known as gene mutations. Gene mutations involving changes in one or a few nucleotides are known as point mutations, because they occur at a single point in the DNA sequence. Point mutations include substitutions, in which one base is changed to another, as well as insertions and deletions, in which a base is inserted or removed from the DNA sequence.
Substitutions usually affect no more than a single amino acid. The effects of insertions or deletions can be much more dramatic. Since the sequences are read in three-base codons, the groupings get shifted when a base is inserted or deleted. Changes like these are called frameshift mutations because they "shift" the reading frame of the sequence. These mutations can alter proteins to the point of be unable to perform its normal purposes.
Chromosomal mutations involve changes in the number or structure of chromosomes. Such mutations may change the locations of genes on chromosomes, and may even change the number of copies of some genes. There are four types of chromosomal mutations.
There are four types of chromosomal mutations: deletions, duplications, inversions, and translocations. Deletions involve the loss of all or part of a chromosome, while duplications produce extra copies of parts of a chromosome. Inversions reverse the directions of parts of chromosomes, and translocations occur when part of one chromosome breaks off and attaches to another.
Significance of Mutations
Many, if not most, mutations have little or no effect. Mutations that cause dramatic changes in protein structure or gene activity are often harmful, producing defective proteins that disrupt normal biological activities. In contrast, beneficial mutations may produce proteins with new or altered activities that can be useful to organisms in different or changing environments.
Significance in Mutations
Mutations in most cells of the body (diploid cells) affect only the individual organism in which they occur- although some of these changes can be dramatic. For example, harmful mutations in body cells cause many forms of cancer. Mutations that occur in cells that produce gametes (sperm & eggs), however, can be passed along to offspring.
The term "mutations" has a negative connotation, however, some mutations are beneficial and breeders will look for these helpful mutations. For example, when a complete set of chromosomes fails to separate during gamete formation. This can result in triploid (3N) or tetraploid (4N) organisms. The condition in which an organism has extra sets of chromosomes is called polyploidy.