In Investigations Two and Three, you examined the inheritance and expression of traits. In Investigation Four, you will take a closer look at the genetic information found in cells and will learn how this information is read and used by the cells. You will also determine that genetic information is translated into proteins which control all of the functions of a cell and all of the traits of an organism.

There are four main types of biological molecules: carbohydrates, proteins, nucleic acids, and lipids. Investigation Four concentrates on two of these – proteins and nucleic acids. Nucleic acids and proteins are both organic molecules since they consist mainly of carbon, oxygen, and hydrogen atoms. Nucleic acids and proteins are also polymers. A polymer is a long, chain-like molecule of similar or identical repeating units called monomers.

Proteins are polymers of amino acids. Proteins are very large biological molecules that control almost all functions of a cell. Proteins provide structure, catalyze chemical reactions, transport materials, regulate functions, and act as signals.

Nucleic acids are polymers of nucleotides. A nucleotide consists of a phosphate group, a pentose (5 carbon) sugar, and a nitrogenous base. There are four types of nucleotides: adenosine (A), cytosine (C), guanosine (G), and thymidine (T). Nucleic acids differ from each other in the type of sugar and the types of nitrogenous bases that they contain. There are two types of nucleic acids: DNA and RNA.

Deoxyribonucleic acid (DNA) contains nucleotides that consist of the sugar deoxyribose and one of the four nitrogenous bases. The four nitrogenous bases found in DNA are pyrimidines: cytosine (C) and thymine (T), and purines: adenine (A) and guanine (G). DNA is a double-stranded nucleic acid consisting of two nucleotide strands. These strands form a double helix with the sugar-phosphate backbones on the outside and nitrogenous base pairs on the inside. The helix is held together by hydrogen bonds.

DNA consists of two strands of nucleotide polymers. Because of the molecular structure of the bases, each nucleotide in one strand will only bond or pair with one of the four nucleotides in the other strand. Adenine always pairs with thymine and cytosine always pairs with guanine. This specific pairing is referred to as being complementary to one another. Because the strands are complementary to one another, they can serve as templates for each other. This characteristic of DNA makes replication, the process by which copies of DNA are made, possible. A brief video showing DNA replication is provided below. Replication is the way one DNA molecule can be exactly copied and used to form two new DNA molecules. The process is much more complicated than shown here. However, the important point to remember is that by making an exact copy of itself, the DNA can be passed to the next generation and therefore is at the very heart of heredity.

The Genetic Material

DNA is often called the genetic material of the cell. Genes are made of DNA. Each gene encodes for one protein. However, genes do not make proteins directly. DNA is located in the nucleus of cells, but protein synthesis occurs in the cytoplasm on structures called ribosomes. The transfer of information from DNA in the nucleus to the ribosomes is mediated by ribonucleic acid (RNA).

Ribonucleic acid (RNA) is made up of nucleotides that contain ribose as the sugar. RNA contains three of the same nitrogenous bases as DNA – cytosine, adenine, and guanine. The fourth base in RNA is the pyrimidine, uracil since RNA does not contain thymine. RNA is single-stranded, consisting of only one polynucleotide chain.

There are three different types of RNA that are involved in protein synthesis. The three types of RNA are mRNA (messenger RNA), tRNA (transfer RNA), and rRNA (ribosomal RNA).

The transfer of information from genes to RNA to protein occurs in two steps, as shown here. The first step is transcription. In this step, the information in DNA is transferred to RNA. The second step is translation. In this process, the information in RNA is translated into a polypeptide (protein).

During transcription, RNA is formed by a protein called RNA polymerase. The polymerase uses one strand of DNA as a template for the RNA. As in the two complementary strands of DNA, the strand of mRNA is complementary to the template DNA strand. For example, the base guanine in the DNA template would correspond to a cytosine in the RNA transcript. Transcription occurs in the nucleus of the cells.

The mRNA leaves the nucleus and is transported to the cytoplasm. Translation occurs on cytoplasmic structures called ribosomes. Translation is the RNA-directed synthesis of proteins. During translation, the mRNA is “read” as a series of three bases called codons, as shown in this figure.

Each codon encodes for a specific amino acid according to a genetic code that is translated from the mRNA by the ribosomes. 

After translation, the protein chain will fold to form its final three-dimensional structure. The proper folding of a protein into its unique structure is essential to its function and is determined by its amino acid monomers.

A mutation is a change in the sequence of nucleotides of a gene that could result in a change in the protein’s structure. A mutation may result in a change in the folding of a protein which can alter the function of that protein. Some proteins that are altered by mutations can result in serious diseases. Other mutations which change a protein’s structure only minimally, or not at all, are not harmful and will allow a protein to function normally. Still other mutations may provide an organism with a selective advantage in a particular environment.

In Investigation Four, you will construct a protein using a set of “RNA” instructions and then test whether the protein will be able to perform its function. You will also have a chance to decode a sequence of nucleotides. You will discover that one mistake in a sequence of nucleotides can cause a disruption in the interpretation of that sequence, which may or may not affect whether that sequence can be read by a cell.