DNA Structure

James Watson and Francis Crick were the first scientists who were able to piece together the structure of DNA and enabled scientists to clarify how DNA determines traits and how traits are passed on from generations. DNA is a long, thin molecule in the shape of a double helix. The two sides of the helix are made of nucleotides which consist of a phosphate group, a five-carbon sugar base molecule, and a nitrogen base. Nucleotides are all identical except for the bases which are adenine, thymine, guanine, and cytosine. Adenine is paired with thymine and guanine is paired with cytosine. The sides are composed of alternating sugar and phosphate molecules and the rungs of the helix are made of pairs of nitrogen bases held together by weak hydrogen bonds. A human cell has 46 chromosomes with more than 5 billion nucleotides. The sequence in which the nucleotides are arranged along the ladder is the genetic code.
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DNA Replication

DNA Replication is what allows cells to build new, exact copies of itself and allows them to pass genetic material to offspring. The hydrogen bonds that connect the complementary bases are broken by enzymes called DNA helicases. Then, proteins hold each strand apart to keep them from returning to their original form. DNA polymerases add nucleotides to the nitrogen bases from the "free floating" nucleotides in the nucleus and two new double helixes begin to form. Two DNA molecules are created each containing one old and one new strand. The DNA polymerases "proofread" each new DNA strand and remove a nucleotide if it is incorrectly paired in order to prevent errors. Only one error per 1 billion nucleotides occurs which is called a mutation. Each human chromosome is replicated in about 100 sections that are 100,000 nucleotides long in about 8 hours.

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1. Who were the scientists that first discovered the structure of DNA?

2. How many chromosomes does each human cell contain?

3. What are the enzymes that break the hydrogen bonds between nitrogen bases?

4. What is genetic code?

5. How long does it take a human chromosome to replicate?