DNA directs the cell’s activities by telling it what proteins to makeand when. These proteins form structural elements in the cell andregulate the production of other cell products. By controlling proteinsynthesis, DNA is hugely important in directing life.
Protein synthesis is a two-step process.DNA resides in the nucleus, but proteins are made in the cytoplasm. Thecell copies the information held in DNA onto RNA molecules in a processcalled transcription. Proteins are synthesized at the ribosomes fromthe codes in RNA in a process called translation.
Before getting into the way that theinformation on DNA can be transcribed and then translated into protein,we have to spend some time studying the major players in the process: DNA and RNA.
DNA and the Genetic Code
The sequence of nucleotides in DNA makesup a code that controls the functions of the cell by telling it whatproteins to produce. Cells need to be able to produce 20different amino acids in order to produce all the proteins necessary tofunction. DNA, however, has only four nitrogen bases. How can thesefour bases code for the 20 aminoacids? If adenine, thymine, guanine, and cytosine each coded for oneparticular amino acid, DNA would only be able to code for four aminoacids. If two bases were used to specify an amino acid, there wouldonly be room to code for 16 (
) different amino acids.
In order to be able to code for 20 amino acids, it is necessary to use three bases (which offer a total of 64 coding combinations) to code for each amino acid. These triplets of nucleotides that make up a single coding group are called
codons or
genes. Two examples of codons are CAG, which codes for the amino acid glutamine, and CGA, which codes for arginine.
Codons are always read in anon-overlapping sequence. This means that any one nucleotide can onlybe a part of one codon. Given the code AUGCA, AUG could be a codon forthe amino acid methionine, with CA starting a new codon. Alternatively,GCA could be a codon specifying alanine, while the initial AU was thelast two letters of a previous codon. But AUG and GCA cannot both becodons at the same time.
Degeneracy of the Genetic Code
There are 64 codons but only 20 amino acids. What happens to the other 44coding possibilities? It happens that some of the different codons callfor the same amino acid. The genetic code is said to be
degenerate because of its redundancy.
Experiments have shown that there are also three
stop codons, which signal when a protein is fully formed, and one start codon, which signals the beginning of an amino acid sequence.
Mutations of the Genetic Code
Since the sequence of nucleotides in DNAdetermines the order of amino acids in proteins, a change or error inthe DNA sequence can affect a protein’s function. These errors orchanges in the DNA sequence are called mutations.
There are two basic types of mutations: substitution mutations and frameshift mutations.
Substitution Mutation
A substitution mutation occurs when a singlenucleotide is replaced by a different nucleotide. The effects ofsubstitution mutations can vary. Certain mutations might have no effectat all: these are called silent mutations. For instance, because thegenetic code is degenerate, if the particular codon GAA becomes GAG, itwill
still code for the amino acid glutamate and the functionof the cell will not change. Other substitution mutations candrastically affect cellular and organismal function. Sickle-cellanemia, which cripples human red blood cells, is caused by asubstitution mutation. A person will suffer from sickle-cell anemia ifhe has the amino acid valine in his hemoglobin rather than glutamicacid. The codon for valine is GUA or GUG, while the codon for glutamicacid is GAA or GAG. A simple substitution of A for U results in thedisease.
Frameshift Mutation
A frameshift mutation occurs when anucleotide is wrongly inserted or deleted from a codon. Both types offrameshifts usually have debilitating or lethal effects. An insertionor deletion will affect
every codon in a particular geneticsequence by throwing the entire three-by-three codon structure out ofwhack. For example, if the A in the GAU were to be deleted, the code:
GAU GAC UCC GCU AGG
would become:
GUG ACU CCG CUA GG
and code for an entirely different set ofamino acids in translation. The results of such mutations on anorganism are usually catastrophic.
The only sort of frameshift mutation thatmight not have dire effects is one in which an entire codon is insertedor deleted. This will result in the gain or loss of one amino acid butwill not affect surrounding codons.
Chromosomes
Even the tiniest cells contain meters uponmeters of DNA. With the aid of special proteins called histones, thisDNA is coiled into an entangled fibrous mass called chromatin. When itcomes time for the cell to replicate (a process covered later in thischapter), these masses gather into a number of discrete compactstructures called chromosomes.
In eukaryotes, the chromosomes are located inthe nucleus of the cell. Prokaryotes don’t have a nucleus: their DNA islocated in a single chromosome that is joined together in a ring. Thisring chromosome is found in the cytoplasm. In this chapter, when wetalk about chromosomes, we will be referring to eukaryotic chromosomes.
Different eukaryotes have varying numbers ofchromosomes. Humans, for example, have 46 chromosomes arranged in 23pairs. (Dogs have 78 chromosomes in 39 pairs. A larger number ofchromosomes is not a sign of
greater biological sophistication.) Thetotal 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
somaticcells, contain chromosomes in two closely related sets—one set of 23each from a person’s mother and father—making up a total of 46chromosomes. 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 numberof 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 somaticcells.
In humans and other higher animals, only thesex cells that are passed on to offspring have the haploid number ofchromosomes. These sex cells are also called
gametes.
RNA
Ribonucleic acid (RNA) helps DNA turn storedgenetic messages into proteins. As discussed in the Biochemistrychapter, RNA monomers (nucleotides) are similar to those of DNA, butwith three crucial differences:
- DNA’s five-carbon sugar is deoxyribose. RNA nucleotides contain a slightly different sugar, called ribose.
- RNA uses the nitrogenous base uracil in place of DNA’s thymine.
- The RNA molecule takes the form of a single helix—half a spiral ladder—as compared with the double helix structure of DNA.
Two different types of RNA play important roles in protein synthesis. During transcription, DNA is copied to make
messenger RNA (mRNA),which then leaves the nucleus to bring its still encoded information tothe ribosomes in the cytoplasm. In order to use the informationcontained in the transcribed mRNA to make a protein, a second type ofRNA is used.
Transfer RNA (tRNA) moves amino acids to the siteof protein synthesis at the ribosome according to the code specified bythe mRNA strand. There are many different tRNAs, each of which bond toa different amino acid and the mRNA sequence corresponding to thatamino acid.
