Chapter 16: The Molecular Basis of Inheritance

Q: Why are the nitrogenous of the double helix paired in unusual, specific combinations, not like-with-like paring?

A: It is because adenine and guanine are purines, nitrogenous bases with two organic rings, and cytosine and thymine belong to the family of nitrogenous known as pyrimidines, which have a single ring.

Q: So how does the Watson-Crink model explain the basis for Chargaff's rule?

A: The Watson-Crink model surely explains it because the model describes how the two pairs of the nitrogenous bases are meant to be because of their formations; therefore it automatically proves Chargaff's rule because they have to be paired up having the same amount with the partner.

Q: What are telomeres?

A: Special nucleotide sequences at the ends of eukaryotic chromosomal DNA molecules that do not contain genes; instead, the DNA typically consists of multiple repetitions of one short nucleotide sequence. They postpone the erosion of genes near the ends.

1. DNA is the genetic material

2. Many proteins work together in DNA replication and repair.

3. A chromosome consists of a DNA molecule packed together with proteins.

4. The four nitrogenous bases that DNA consist are Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).

5. DNA polymerases can add nucleotides only to the free 3'end of a primer or growing DNA strand, never to the 5'end.

Figure: 16.8

The Pairs of nitrogenous bases in a DNA double helix are held together by hydrogen bonds, shown here as pink dotted lines.

By the 1940’s scientists knew that chromosomes carry the hereditary meterial. DNA apeared to be a much simpler molecule about whch little was known, Therefore, in 1940 almost all scientists thought that proteins must be responsible for inheritance.

By 1944 scientists however knew that DNA was the hereditary material as a result of an experiment by Frederick Griffiths in the 1920’s. Watson and Crick discovered the double helix by building models to conform to X-ray data.

The replication of a DNA molecule begins at special sites, origins of replication. In bacteria, this is a specific sequence of nucleotides that is recognized by the replication enzymes. As each nucleotide is added to the growing end of a DNA strand, the last two phosphate groups are hydrolyzed to form pyrophosphate.

Each DNA strand has a 3’ end with a free hydroxyl group attached to deoxyribose and a 5’ end with a free phosphate group attached to deoxyribose.