Replication Duplication of Information

DNA must duplicate itself precisely in order to pass along its information from generation to generation. DNA replication occurs during the S phase of the cell cycle (see Chapter 3). Differences between the DNA of one organism and that of another lie in the sequence of the four possible types of ladder rungs and their total number. If the four nucleotides are synthesized in living cells, then what tells the cell exactly how to put them together to form DNA molecules with proper nucleotide sequences?

Watson and Crick observed that if the two strands of DNA are "unzipped" by an enzyme breaking the hydrogen bonds between the nucleotides down the middle of the ladder,

Chapter 13

Dna Unzip Enzyme

Figure 13.4 Replication (duplication) of DNA. A molecule replicates by "unzipping" along the hydrogen bonds, which link the pairs of nucleotides in the middle, and then each half serves as a template for nucleotides available in the cell. As the new nucleotides fit into the appropriate places and are linked together, a double helix is re-formed for each original half.

Figure 13.4 Replication (duplication) of DNA. A molecule replicates by "unzipping" along the hydrogen bonds, which link the pairs of nucleotides in the middle, and then each half serves as a template for nucleotides available in the cell. As the new nucleotides fit into the appropriate places and are linked together, a double helix is re-formed for each original half.

then each separated chain provides all the information needed to put together a new ladder (Fig. 13.4). In other words, since guanine can pair only with cytosine, and vice versa, a guanine nucleotide synthesized by the cell can lock on, or bond, only to a thymine nucleotide. If the two strands of a DNA molecule "unzip," then individual nucleotide "building blocks" can line up next to each of the single chains in precise sequence and be bonded together to form a new chain; each single chain acts as a mold or template for the creation of a new double-stranded DNA molecule. When replication is complete, two double helices have been created from a single one. Each new DNA molecule consists of one strand from the original molecule and another built using that parental strand as a template. This is called semi-conservative replication.

Replication is a remarkably rapid and precise process. Nucleotides are added by the enzyme DNA polymerase at a rate of approximately 33 bases per second during DNA replication. However, even at this rate, a chromosome that is, for example, 200 million base pairs long, would take 70 days to replicate. Because a cell must replicate its DNA before it undergoes mitosis, this replication rate is far too slow for normal plant growth. How is this problem resolved for the cells? In two ways: (1) replication proceeds simultaneously in both directions on the chromosome, cutting the replication time in half; and (2) replication begins at several points on a chromosome nearly simultaneously.

The nucleus contains enzymes that are capable of "proofreading" during replication. They detect and correct mismatched base pairs, such as guanine paired with adenine or thymine paired with cytosine. Consequently, about only one in every million nucleotides added during replication is incor rectly paired. However, each nucleus of a corn plant contains 4 billion base pairs of DNA. Therefore, about 4,000 mutations (changes in DNA sequence) occur with each cell division.

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Responses

  • claudia
    How do i pair duplication dna?
    8 years ago
  • REZENE
    What enzyme puts new nucleotides across from each half of the dna molecule?
    8 years ago
  • maire
    What enzyme helps unzip the dna molecule for replication?
    8 years ago

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