A substitution mutation occurs when a single nucleotide is replaced by a different nucleotide. The effects of substitution mutations can vary. Certain mutations might have no effect at all: these are called silent mutations. For instance, because the genetic code is degenerate, if the particular codon GAA becomes GAG, it will still code for the amino acid glutamate and the function of the cell will not change. Other substitution mutations can drastically affect cellular and organismal function. Sickle-cell anemia, which cripples human red blood cells, is caused by a substitution mutation. A person will suffer from sickle-cell anemia if he has the amino acid valine in his hemoglobin rather than glutamic acid. The codon for valine is GUA or GUG, while the codon for glutamic acid is GAA or GAG. A simple substitution of A for U results in the disease.

 

    Frameshift Mutation

 

    A frameshift mutation occurs when a nucleotide is wrongly inserted or deleted from a codon. Both types of frameshifts usually have debilitating or lethal effects. An insertion or deletion will affect every codon in a particular genetic sequence by throwing the entire three-by-three codon structure out of whack. For example, if the A in the GAU were to be deleted, the code:

would become:

    and code for an entirely different set of amino acids in translation. The results of such mutations on an organism are usually catastrophic.

 

    The only sort of frameshift mutation that might not have dire effects is one in which an entire codon is inserted or deleted. This will result in the gain or loss of one amino acid but will not affect surrounding codons.

 

 

    Even the tiniest cells contain meters upon meters of DNA. With the aid of special proteins called histones, this DNA is coiled into an entangled fibrous mass called chromatin. When it comes time for the cell to replicate (a process covered later in this chapter), these masses gather into a number of discrete compact structures called chromosomes.

In eukaryotes, the chromosomes are located in the nucleus of the cell. Prokaryotes don’t have a nucleus: their DNA is located in a single chromosome that is joined together in a ring. This ring chromosome is found in the cytoplasm. In this chapter, when we talk about chromosomes, we will be referring to eukaryotic chromosomes.

 

    Different eukaryotes have varying numbers of chromosomes. Humans, for example, have 46 chromosomes arranged in 23 pairs. (Dogs have 78 chromosomes in 39 pairs. A larger number of chromosomes is not a sign of greater biological sophistication.) The total number of distinct chromosomes in a cell is the cell’s diploid number.

 

    The cells in a human body that are not passed down to offspring, called somatic cells, contain chromosomes in two closely related sets—one set of 23 each from a person’s mother and father—making up a total of 46 chromosomes. These sets pair up, and the pairs are known as homologous chromosomes. Each homologous pair consists of one maternal and one paternal chromosome. The haploid number of a cell refers to half of the total number of chromosomes in a cell (half the diploid number), or the number of homologous pairs in somatic cells.