This video describes nucleotides, ribose and deoxyribose, DNA and RNA structure and function. Downloadable worksheet available.
NUCLEIC ACIDS
Nucleic acids provide cellular “instructions” that determine the correct amino acid sequence for every protein produced by every cell in the body. This is true for every living organism.
DNA
Each cell in the human body contains a nucleus with 46 chromosomes. We have two copies of 23 chromosomes: one set received from each parent. Each chromosome consists of one nucleic acid macromolecule called DNA: deoxyribonucleic acid. Each DNA macromolecule contains hundreds to thousands of genes, which give the blueprint (instructions) for the production of a specific protein. Large chromosomes can have over 2000 genes, such as chromosome 1. Smaller chromosomes, such as chromosome 21, have less than 400 genes (Figure 2.11). The X chromosome has over 2000 genes, and the male Y chromosome has only 78 genes. In total, the human genome is made up of 25 000 genes.
Did You Know?
A single DNA molecule, if stretched to its full length, is approximately two inches or 50 mm long? All 46 chromosomes must be extremely condensed to fit into each cell nucleus, which is only about 10 micrometres in diameter!
DNA Structure
As discussed in Section 2.3, the order of amino acids in the protein determines the structure and function of each of the thousands of proteins in the human body. Every cell produces the correct proteins with the correct order of amino acids because of the sequence of nucleotides in each gene. When red blood cells produce the hemoglobin protein, the order of amino acids is determined by the nucleotides in the hemoglobin gene, which all humans have on chromosome 11. Exactly how nucleotide sequences lead to amino acid sequences is discussed in detail in Chapter 8. Each DNA nucleotide is an organic monomer made up of three components: a five-carbon sugar (deoxyribose), a phosphate group, and a nitrogen-containing base (Figure 2.12a). Adenine and guanine are nitrogenous bases with a double-ring structure; they are called purines. Cytosine and thymine are single-ring bases called pyrimidines (Figure 2.12b).
It is often said that DNA contains the “stored information”— or is the “blueprint”—for protein synthesis. This is because it carries the specific order of the nucleotides (A, T, C, and G) in every gene, and this translates into the amino acid sequence of every protein that’s synthesized. Imagine that our nucleotides are like letters of the alphabet; from letters, we make words. The sequence of nucleotide “letters” has to be correct in order to spell amino acid “words” accurately and to make the proper protein “sentences.” A misspelled word can dramatically change the meaning of a sentence. Such a misspelling of an amino acid sequence corresponds to a DNA mutation.
Did You Know?
Some genes are expressed only at certain times in our life, such as the gene for fetal hemoglobin, which is useful for transporting oxygen in a fetus. After birth, the red blood cells make a different hemoglobin protein from a different gene.
DNA is a double helix whereby two strands of DNA bind together by the nitrogenous bases. A gene can be as small as 100 nucleotides, or as long as 2000 nucleotides on one strand of DNA. On each strand of DNA the sugar and the phosphates are bound together by very strong phosphodiester bonds; these make up the “backbone” of the DNA strand and the nitrogenous bases extend from each sugar molecule. The sugar is bound to the base by a covalent glycosidic bond, also very strong. Glycosidic bonds are found between any sugar and some other group that could be another sugar or something else, such as a nitrogenous base. Each base binds with only one specific base on the opposite strand. Adenine always binds with thymine, and cytosine always binds with guanine (Figure 2.13). The nucleotide bases bind to the opposite complementary nucleotide by weak hydrogen bonds. Two hydrogen bonds form between adenine and thymine, and three hydrogen bonds hold guanine and cytosine together. The two strands of DNA are antiparallel because they are oriented in opposite directions. Note that each carbon on the deoxyribose sugar shown in Figure 2.13 is numbered. The first carbon is bound to the base, and the third and fifth carbons are bound to phosphate groups. The DNA strand on the left has a free phosphate group attached to carbon number 5: this is called the 5’ (five prime) end. The opposite strand has a free phosphate on carbon number 3: this is called the 3’ (three prime) end. How DNA polymerase enzymes replicate DNA molecules is covered in Chapter 8.
RNA
The other type of nucleic acid macromolecule found in cells is called RNA: ribonucleic acid. The function of DNA and RNA is quite different. RNA is the intermediate step between the nucleotide sequence of each gene and the eventual completed protein. Suppose that a pancreas cell is stimulated by increased blood sugar levels to produce the protein hormone insulin. Inside each pancreatic beta islet cell nucleus, the insulin gene sequence becomes transcribed into an RNA sequence (Figure 2.14). That RNA molecule then leaves the nucleus and moves into the cytoplasm of the cell to a ribosome, where it acts as the template for the assimilation of the amino acid sequence that produces insulin. The insulin gene consists of 153 nucleotides that lead to the production of the small, 51-amino acid, insulin protein. The process of RNA transcription is discussed in detail in Chapter 9 (see Table 2.3).
