Biology DNA structure and DNA replication

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Biology DNA structure and DNA replication

Topic 2.4 - DNA Structure

2.4.1. Outline DNA nucleotide structure in terms of sugar (deoxyribose), base and phosphate.

A DNA nucleotide is composed of deoxyribose, a phosphate group and a nitrogenous base (adenine, guanine, thymine, or cytosine). The phosphate group is covalently bonded to the carbon of the deoxyribose, and the nitrogenous base is attached to the deoxyribose on the opposite side.

2.4.2. State the names of the four bases of DNA.

Adenine, Guanine, Thymine, and Cytosine.

2.4.3. Outline how the DNA nucleotides are linked together by covalent bonds into a single strand.

Drawing will be inserted at a later date.

2.4.4. Explain how a DNA double helix is formed using complimentary base pairing and hydrogen bonds.

Each sugar of the backbone (sides of the "ladder") is covalently bonded to a nitrogenous base. Each of these bases forms hydrogen bonds with its complimentary nitrogenous base, forming the '"rungs" of the "ladder". The sides of the ladder are composed of alternating sugar and phosphate groups. The rungs are each composed of two nucleotides which are attached to the sugars of opposite sides of the DNA ladder and are attatched to eachother by hydrogen bonds.

2.4.5. Draw a simple diagram of the molecular structure of DNA.

Drawing will be inserted at a later date.

Topic 2.5 - DNA Replication

2.5.1. State that DNA replication is semi-conservative.

DNA is semi-conservative.

2.5.2. Explain DNA replication in terms of unwinding of the double helix and separation of the strands by helicase, followed by formation of the new complementary strands by DNA polymerase.

When replication takes place, the enzyme helicase first unwinds the double helix . Next the two DNA strands are split apart at hundreds, sometimes thousands, of points along the strand. Each splitting point is an area where replication is occuring, called a replication bubble. In each replication bubble, new DNA is made by attaching free nucleotides to the original strand (called the template) by base-pairing rules with the help of the enzyme DNA polymerase. The process results in two identical DNA strands produced from one.

2.5.3. Explain the significance of complementary base pairing in the conservation of the base sequence of DNA.

Because the nitrogenous bases that compose DNA can only pair with complementary bases, any two linked strands of DNA are necessarily complementary to one another. The fact that only complementary base pairs can join together means that in replication the newly formed strands must be complementary to the old strands, thus conserving the same base sequence as previously existed.

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Topic 2.6 - Transcription and Translation

2.6.1. Compare the structure of RNA and DNA.

RNA has the ribose sugar while the DNA has the deoxyribose sugar in its structure. RNA is only one single strand while DNA has a double helix with two strands. Also, the thymine nucleotide of DNA is replaced by uracil in RNA (uracil, like thymine, attaches to adenine by hydrogen bonds).

2.6.2. Outline the DNA transcription in terms of the formation of an RNA strand complementary to the DNA strand by RNA polymerase.

The synthesis of RNA uses DNA as a template. First, the two strands of DNA are separated in a specific place. Then, with the help of RNA polymerase, RNA nucleotides attach to thier complimentary bases on one side of the exposed DNA strand. This creates a single strand of complimentary nucleotide bases. After this is done, the RNA molecule separates from the DNA.

2.6.3. Describe the genetic code in terms of codons composed of triplets of bases.

The genetic code for an amino acid is contained in DNA as a series of three nitrogenous bases. Each of these triplets (codons) code for a particular amino acid.

2.6.4. Explain the process of translation, leading to peptide linkage formation.

After transcriptions, the mRNA moves out of the nucleus into the cytoplasm where the mRNA attaches ro a ribosome. In the cytoplasm there are transfer RNA (tRNA) molecules. These molecules are composed of a short RNA molecule folded into a specific shape. Each tRNA molecule is shaped so that it bonds to a certain amino acid. Each tRNA moelcule also has an anticodon which compliments a certain mRNA codon. Once the mRNA attaches to a ribosome, it acts as a sort of conveyor belt. The tRNA molecules attach to the mRNA according to the complimentary nature of their bases. For example, a tRNA molecule with the anitcodon ACC will carry the amino acid tryptophan. This tRNA molecule will attach to the codon UGG on the mRNA because UGG compliments ACC. After two tRNA molecules are attached to the mRNA, they bond and the first tRNA molecule is released. Then another tRNA molecule connects to the mRNA etc, and the polypeptide is created.

2.6.5. Define the terms degenerate and universal as they relate to the genetic code.

Degenerate means that multiple triplets code for the same amino acid. For example, UUU and UUC both code for phenylalanine. Univeral refers to the fact that this genetic code occurs in all living organisms.

2.6.6. Explain the relationship between one gene and one polypeptide.

One gene corresponds to one polypeptide. It does not, however, always code for a protein, because many proteins consists of more than one polypetide.

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