Expectations[]
- Describe and explain the process of DNA replication.
- Explain how genetic technologies were developed to copy DNA.
Notes[]
DNA replication
The process of a cell uses to copy its chromosomes is called DNA replication. During DNA replication two DNA molecules are made from one, each "daughter" molecule identical to, and inheriting a strand from, the "parent" molecule. Since the nucleotides on one strand of DNA pair with the complementary nucleotides on the other strand, each strand can serve as a template to forming a new complementary strand.
The first step in DNA replication is when the enzyme helicase breaks the hydrogen bonds between the nitrogen bases, "unzipping" the double helix. As the DNA splits, another enzyme, DNA polymerase, moves along each strand attaching free nucleotides to each base until the original molecule has been entirely unzipped and two new molecules are formed. Each new DNA molecule is identical to the parent molecule, barring any errors.
Errors in Replication[]
Mistakes, or mutations, occasionally occur during DNA replication. The number of mutations passed to new cells is small because enzymes "proofread" the new strands of DNA and can sometimes, but not always, correct mistakes.
A mutation may actually benefit the organism (positive mutation), harm it (negative mutation) or have no effect (neutral mutation). A mutation in a somatic cell of an organism usually has a much less drastic effect than a mutation in a gamete or growing embryo. A low rate of mutation is present in every population.
External influences such as radiation or exposure to mutagenic chemicals can cause mutations. Such mutagens increase the natural rate of mutation and can alter the DNA and affect its replication. As a result a different protein or property of a protein may develop which could adversely affect the cell. For example, some mutations cause cells to lose control of the cell division process and to grow abnormally. The cells reproduce in an uncontrolled manner and are said to be cancerous. These cells can spread out, crowding healthy cells and destroying tissues into which they have spread. Any body cell can behave in this manner so there are many different types of cancer depending on which type of cell is affected.
Mutagens include gases such as Agent Orange and mustard gas. The most widespread is ultraviolet (UV) radiation from the Sun. The energy from the light is absorbed by DNA and can cause the hydrogen bonds of base pairs to break apart and fuse to adjacent bases. This causes inaccurate replication and abnormal cell division. Eventually, tumours and skin cancer can result.
Base pair mutations occur when one base pair is replaced by another base pair in a DNA strand. This type of change may alter the code and product a different amino acid, resulting in a different polypeptide chain. This type of mutation can also cause problems if the substitution produces a stop codon which would halt the formation of a polypeptide chain before completion.
A frameshift mutation occurs when one or more base pairs are added or deleted from a DNA strand. This shifts all the nucleotides after the point of mutation up or down moving out of their usual "frame" causing all the codons from that point to code for different amino acids.
Cloning[]
Cloning is the production of identical copies of molecules, genes, cells, or whole organisms. Dolly the sheep was cloned by Scottish scientist Ian Wilmut in 1997. Wilmut took an egg cell from one adult female sheep and removed its nucleus. He then replaced the nucleus with the nucleus from a mammary gland cell of a different adult female sheep. The egg cell was then implanted into the uterus of a surrogate mother sheep and the cell began to form an embryo. Five months later Dolly was born, an exact genetic replica of the sheep that had donated the nucleus.
More common than cloning entire organisms is cloning genes. In gene cloning, multiple copies of DNA are produced. Scientists can copy specific sections of DNA and insert them into the chromosomes of bacteria cells. Then, when the bacterial cell divides and replicates its DNA, the inserted section of DNA will also be copied. This allows DNA to be copied rapidly since bacteria cells reproduce quickly. One example of this technique is the production of human insulin.
Gene cloning is one way to create millions of copies of a single gene, but is not the only way to clone DNA. A polymerase chain reaction (PCR) can do a similar thing. However, PCR can copy a DNA sequence that is a millionth of the total DNA sample, meaning a gene or smaller piece of DNA can be copied. PCR can be used to amplify, or generate multiple copies of, DNA from crime scenes. DNA from mummies and fossils can be amplified and studied using PCR.