pumping of the heart. Filtration occurs when blood flows through capillaries, whose walls are only one cell thick and very permeable. The blood pressure in capillaries is higher than the pressure of the surround- ing tissue fluid. In capillaries throughout the body, blood pressure forces plasma (water) and dissolved materials through the capillary membranes into the surrounding tissue spaces (see Fig. 3–3). This cre- ates more tissue fluid and is how cells receive glu- cose, amino acids, and other nutrients. Blood pressure in the capillaries of the kidneys also brings about filtration, which is the first step in the formation of urine.
PHAGOCYTOSIS AND PINOCYTOSIS
These two processes are similar in that both involve a cell engulfing something, and both are forms of endo- cytosis, endomeaning “to take into” a cell. An exam- ple of phagocytosis is a white blood cell engulfing bacteria. The white blood cell flows around the bac- terium (see Fig. 3–3), taking it in and eventually digesting it. Digestion is accomplished by the enzymes in the cell’s lysosomes.
Other cells that are stationary may take in small molecules that become adsorbed or attached to their membranes. The cells of the kidney tubules reabsorb small proteins by pinocytosis (see Fig. 3–3) so that the protein is not lost in urine.
Table 3–2 summarizes the cellular transport mech- anisms.
57 Cell nucleus
Nuclear pore
Nuclear membrane
A
A Codon
A DNA
Amino acids
Peptide bonds
mRNA
tRNA
Ribosome
Anticodon A
A G
C A U A A A G U C U U U
G
U A
U U U
Figure 3–4. Protein synthesis. The mRNA is formed as a copy of a portion of the DNA in the nucleus of a cell. In the cytoplasm, the mRNA becomes attached to ribosomes. See text for further description.
QUESTION:A tRNA molecule has two attachment sites; what is each for?
Recall that in Chapter 2 you read that a geneis the genetic code for one protein. This is a simplification, and the functioning of genes is often much more com- plex. We have genes with segments that may be shuf- fled or associated in many combinations, with the potential for coding for many more proteins. A full explanation is beyond the scope of our book, so for the sake of simplicity, and in the following discussion, we will say that a gene is the code for one protein. Recall too that a protein is a specific sequence of amino acids.
Therefore, a gene, or segment of DNA, is the code for the sequence of amino acids in a particular protein.
The code for a single amino acid consists of three bases in the DNA molecule; this tripletof bases may be called a codon(see Fig. 3–4). There is a triplet of bases in the DNA for each amino acid in the protein.
If a protein consists of 100 amino acids, the gene for that protein would consist of 100 triplets, or 300 bases.
Some of the triplets will be the same, since the same amino acid may be present in several places within the protein. Also part of the gene are other triplets that start and stop the process of making the protein, rather like capital letters or punctuation marks start and stop sentences.
RNA AND PROTEIN SYNTHESIS
RNA, the other nucleic acid, has become a surprising molecule, in that it has been found to have quite a few functions. It may be involved in the repair of DNA, and it is certainly involved in gene expression. The expression of a gene means that the product of the gene is somehow apparent to us, in a way we can see or measure, or is not apparent when it should be.
Examples would be having brown eyes or blue eyes, or having or not having the intestinal enzyme lactase to digest milk sugar. Although these functions of RNA are essential for us, they too are beyond the scope of our book, so the roles of RNA in the process of pro- tein synthesis will be our focus.
The transcription and translation of the genetic code in DNA into proteins require RNA. DNA is found in the chromosomes in the nucleus of the cell, but protein synthesis takes place on the ribosomes in the cytoplasm. Messenger RNA (mRNA) is the intermediary molecule between these two sites.
When a protein is to be made, the segment of DNA that is its gene uncoils, and the hydrogen bonds between the base pairs break (see Fig. 3–4). Within the nucleus are RNA nucleotides (A, C, G, U) and enzymes to construct a single strand of nucleotides that is a complementary copy of half the DNA gene
(with uracil in place of thymine). This process is tran- scription, or copying, and the copy of the gene is mRNA, which now has the codons for the amino acids of the protein, and then separates from the DNA. The gene coils back into the double helix, and the mRNA leaves the nucleus, enters the cytoplasm, and becomes attached to ribosomes.
As the copy of the gene, mRNA is a series of triplets of bases; each triplet is a codon, the code for one amino acid. Another type of RNA, called transfer RNA (tRNA), is also found in the cytoplasm. Each tRNA molecule has an anticodon, a triplet comple- mentary to a triplet on the mRNA. The tRNA molecules pick up specific amino acids (which have come from protein in our food) and bring them to their proper triplets on the mRNA. This process is translation; that is, it is as if we are translating from one language to another—the language of nucleotide bases to that of amino acids. The ribosomes contain enzymes to catalyze the formation of peptide bonds between the amino acids. When an amino acid has been brought to each triplet on the mRNA, and all peptide bonds have been formed, the protein is finished.
The protein then leaves the ribosomes and may be transported by the endoplasmic reticulum to wherever it is needed in the cell, or it may be packaged by the Golgi apparatus for secretion from the cell. A sum- mary of the process of protein synthesis is found in Table 3–3.
Thus, the expression of the genetic code may be described by the following sequence:
Each of us is the sum total of our genetic charac- teristics. Blood type, hair color, muscle proteins, nerve cells, and thousands of other aspects of our structure and functioning have their basis in the genetic code of DNA.
If there is a “mistake” in the DNA, that is, incorrect bases or triplets of bases, this mistake will be copied by the mRNA. The result is the formation of a malfunc- tioning or non-functioning protein. This is called a geneticor hereditary disease, and a specific example is described in Box 3–2: Genetic Disease—Sickle-Cell Anemia.
DNA RNA Proteins:
Structural Enzymes
Catalyze Reactions Hereditary Characteristics Proteins
58 Cells
59
Normal hemoglobin
Normal red blood cells (RBCs)
Sickle red blood cells Sickle hemoglobin
(HbS) Deo
xygenatio
n
α
β β
α
Iron in heme
Box Figure 3–B Structure of hemo- globin A and sickle-cell hemoglobin and their effect on red blood cells.
A genetic disease is a hereditary disorder, one that may be passed from generation to generation.
Although there are hundreds of genetic diseases, they all have the same basis: a mistake in DNA.
Because DNA makes up the chromosomes that are found in eggs and sperm, this mistake may be passed from parents to children.
Sickle-cell anemia is the most common genetic disorder among people of African descent and affects the hemoglobin in red blood cells. Normal hemoglobin, called hemoglobin A (HbA), is a pro- tein made of two alpha chains (141 amino acids each) and two beta chains (146 amino acids each).
In sickle-cell hemoglobin (HbS), the sixth amino acid in each beta chain is incorrect; valine instead of the glutamic acid found in HbA. This difference seems minor—only 2 incorrect amino acids out of more than 500—but the consequences for the per- son are very serious.
HbS has a great tendency to crystallize when oxygen levels are low, as is true in capillaries. When HbS crystallizes, the red blood cells are deformed into crescents (sickles) and other irregular shapes.
These irregular, rigid red blood cells clog and rup- ture capillaries, causing internal bleeding and severe pain. These cells are also fragile and break up easily, leading to anemia and hypoxia (lack of oxy- gen). Treatment of this disease has improved greatly, but it is still incurable.
What has happened to cause the formation of HbS rather than HbA? Hemoglobin is a protein; the gene for its beta chain is in DNA (chromosome 11).
One amino acid in the beta chains is incorrect, therefore, one triplet in its DNA gene must be, and is, incorrect. This mistake is copied by mRNA in the cells of the red bone marrow, and HbS is synthe- sized in red blood cells.
Sickle-cell anemia is a recessive genetic disease, which means that a person with one gene for HbS and one gene for HbA will have “sickle-cell trait.”
Such a person usually will not have the severe effects of sickle-cell anemia, but may pass the gene for HbS to children. It is estimated that 9% of African-Americans have sickle-cell trait and about 1% have sickle-cell anemia.