Exercise 2-9: Biology of the Gene

The Genetic Material

During the 20th century, scientists were discovering that the genetic material was DNA and they were trying to describe the structure of it. At this time they knew three things about the genetic material: it is able to store information, it must be stable because it had to be replicated, and it is able to undergo rare changes known as mutations.

Frederick Griffith in the 1920’s determined that bacterial cells can be capsulated or noncapsulated. This meant that some bacteria had a mucus coat that allowed them to be resistant to antibodies and other things. The capsulated bacteria he called the S strain and the noncapsulated he called the R strain. He injected mice with both strains and the ones that were injected with the S strain died and the ones with the R strain did not. But was the capsule alone responsible for the death of the mice? To prove this hypothesis he injected the mice with a heat-killed S strain and the mice did not die. He then injected the mice with both the heat-killed S strain and the R strain and the mice died. He came to the conclusion that the R strain somehow had undergone a transformation that made them capsulated.Transformation occurs when an organism receives foreign DNA and thereby acquires new characteristics.

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The Structure of DNA

In the 1950’s Erwin Chargaff analyzed the base contents of DNA and found that DNA contains four different types of nucleotides: guanine, adenine, cytosine, and thyamine. A nucleotide is composed of a pentose sugar, phosphate group, and a nitrogenous base. The nucleotides can be classified into two groups: purines and pyrimidines. The purines are made of two carbon rings. These nucleotides are guanine and adenine. The pyrimidines are made of a single carbon ring and include cytosine and thyamine. (How to remember? PUGA 2: Purines = guanine, adenine and they have 2 carbon rings) In organisms, the amount of A, T, G, and C is not the same and varies from species to species. But however the amount of A is always equal to the amount of T and the amount of G is always equal to the amount of C. This is known as Chargaff’s Rules. So with the number of A equal to the number of T then they will always bond together and the same with C and G. This is known as complimentary base pairing.

Rosalind Franklin studied DNA using X-Rays and showed that DNA is a helix and is double stranded.

In 1953 James Watson and Francis Crick won a nobel prize for making a model of DNA. This help prove that DNA is a polymer of nucleotides. The nucleotides are joined together by covalent bonds between the phosphate group of one and the pentose sugar of the next. The two strands of DNA are linked by hydrogen bonds. They helped prove complimentary base pairing and

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proved that the strands of DNA are antiparallel meaning that the sugar phosphate groups are oriented in different directions.

DNA Replication

DNA replication is the process of making a copy of a DNA molecule. During replication each old strand of DNA from the parent serves as a template for a new strand in the

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daughter molecule. There are three steps to the process: unwinding, complimentary base pairing, and joining. During the unwinding stage the old strands are unwound and the hydrogen bonds holding them together are broken by a special enzyme called helicase. Then complimentary base pairing occurs. The nucleotides are in the nucleus and are positioned by this process. The last stage is the joining stage where the complimentary nucleotides join together to form new strands. The daughter molecules then have an old and a new strand. This is known as semi-conservative replication. The second and last steps are carried out by an enzyme known as DNA polymerase. Once DNA replication has completed then the cell may divide.

Prokaryote v. Eukaryote Replication

Replication begins at a point in the DNA molecule known as the origin of replication. In eukaryotes there are many origins. Replication bubbles appear and spread until they meet one another. The v shape in the molecule is where the DNA is being replicated at that moment. This

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is known as the replication fork. In eukaryotes the base pairs replicate at about 500-5000 per minute. Mistakes are made about every 1 out of 100,000 base pairs. Over many cell divisions this can impose a problem. DNA polymerase is always proofreading the new strands. If it finds a mistake it takes out the nucleotide and replaces it with the correct one.

Life’s Genetic Code

A gene is a segment of DNA that specifies the sequence of amino acids in a polypeptide of a protein. The central dogma is that the DNA sequence of nucleotides codes for the sequence of amino acids which codes for a specific protein which give the organisms’ structure and phenotype.

RNA is ribonucleic acid. It like DNA is composed of nucleotides. Unlike DNA its nucleotides include guanine, adenine, cytosine and uricil instead of thyamine. It is found in the nucleus and the cytoplasm and is single stranded.

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There are three classes of RNA: messenger RNA (mRNA) which takes messages from the nucleus to the ribosomes located in the cytoplasm; ribosomal RNA (rRNA) which helps make up the ribosome; transfer RNA (tRNA) which transfers amino acids to the ribosomes.

The sequence of nucleotides in DNA specifies the order of amino acids in a polypeptide. There are four nucleotides (U, A, G, C) and twenty amino acids. Each three base pairs code for one amino acid (UGA=1 amino acid). Each set is known as a codon. There are 64 possible codons.

Below is a list of all the possible codons and their amino acids.

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The genetic code is…

1. Degenerate: most amino acids have more than one codon.

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2. Unambiguous: each codon has only one meaning.3. It has one start codon and three stop codons.4. Universal: it suggests a link between all living things.

Transcription and Translation

During transcription the DNA unzips and the mRNA bases attach to the DNA. Then RNA polymerase joins the nucleotides together and the mRNA leaves the nucleus and heads toward the ribosome. The DNA then zips back up.

Translation occurs at the ribosome. The mRNA codons code for the sequence of amino acids. In translation, tRNA brings the right amino acid to the ribosome and it has one end that is an anticodon. There are three steps to translocation: initiation, elongation, and termination. The process is started by the start codon, AUG. In elongation the amino acids are added one at a time to the growing polypeptide chain. During termination the ribosome separates and the polypeptide is released.

Directions: Give the complimentary bases for the following strand of DNA.

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1. Base pairs:

2. Would the new strand run 3’ to 5’ or 5’ to 3’? Why?

Directions: Give the complimentary RNA strand for the following strand of DNA and give the proteins the first two codons code for.

3. RNA base pairs:

4. Proteins for the first two codons:

5. Would the new RNA strand run 5’ to 3’ or 3’ to 5’? Why?

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