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Essential Cell Biology. Chapter 6.

Several illustrations on this page have been taken from the Essential Cell Biology textbook (Garland Publishing, 1998)

DNA strand is composed of four types of nucleotides, A, G, C and T. Each nucleotide is composed of a sugar moiety to which are attached a phosphate group and a nitrogen-containing base.

Nucleotides are linked in a chain through the sugars and phosphates

Nucleotides are assymmetric. They are always linked into a DNA chain in a "head-to-tail" fashion. Therefore, the entire DNA strand has a chemical polarity.

DNA molecule is double stranded. Two strands of DNA are held together by hydrogen-bonding between the bases of nucleotides in different strands. A always pairs with T and G with C.

Each hydrogen bond is weak, but many of them hold two DNA strands together very tightly.

To form Watson-Crick base pairs, DNA strands must be antiparallel.

During DNA replication, each DNA strand is used as a template to synthesize the second DNA strand. DNA strand is ALWAYS synthesized in the 5' to 3' direction.

Chemistry of DNA replication

As a result, the whole DNA molecule is duplicated.

DNA replication is semi-conservative: in the "next generation" molecule one strand is "old" and another is "new"

DNA replication starts within a special region of DNA called REPLICATION ORIGIN which is defined by a specific nucleotide sequence

Replication of DNA is bidirectional. Two Y-shaped replication forks are moving in the opposit directions during DNA replication


The enzyme that synthesizes DNA, DNA polymerase, is self-correcting: it has a proofreading activity

It is not trivial to replicate both DNA strands in the 5' to 3' direction!

DNA synthesis on one of the template DNA strands is discontinuous: new DNA is synthesized in small pieces which then are connected together

DNA synthesis always starts with a RNA primer

Many enzymes are involved in DNA replication

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RNA polymerase, which is responsible for transcription, and ribosome, which is responsible for translation are very accurate enzymes and make very few mistakes. However, if RNA polymerase or a ribosome make a mistake it is not highly detrimental to the cell because only a few mRNA or protein molecules will be affected.

An alteration in the nucleotide sequence of the gene is called a mutation.In contrast to the mistakes made by RNA polymerase or the ribosome, mutations in the gene affect the structure, and therefore, the function of all the proteins expressed from the mutant gene.



1. Synonymous mutation: nucleotide sequence is changed, protein sequence is the same. Since genetic code is degenerate, different codons can code for the same amino acid. Synonymous mutations do not affect functioning of the gene product.


2. Missense mutations: substitution of a nucleotide in the gene sequence that results in substitution of an amino acid in the encoded protein. Nonsense mutations may or may not affect the functioning of the gene product


3. Nonsense mutations: substitution of a nucleotide in the gene sequence that results in generation of a stop codon. Only truncated protein can be produced from the mutant gene. Nonsense mutations usually lead to the production of inactive gene product.


4. Frame-shift mutations: addition or deletion of a nucleotide in the gene sequence results in a change in the reading frame. A significant part of the encoded protein can have an incorrect amino acid sequence. Frame-shift mutations usually lead to the production of an inactive gene product.