Cells and Heredity — From DNA to Protein

1.Cells — the basic unit of life

Every living thing is made of one or more cells. Sometimes a single cell is a whole organism, as with bacteria; sometimes enormous numbers of cells come together to form tissues, organs, and organ systems, as in humans. Most cells are too small to see with the naked eye, so they are observed under a microscope.

Inside a cell are structures, each with its own job (organelles). Plant cells have a cell wall, chloroplasts, and a large vacuole, which animal cells lack. These structures are what let plants stand firm and make food from sunlight.

2.DNA, genes, and chromosomes

DNA is a long molecule that carries genetic information, with a double-helix structure of two twisted strands. Along each strand runs a sequence of four kinds of bases — A (adenine), T (thymine), G (guanine), and C (cytosine) — and the bases facing each other pair up only as A with T and G with C. The order of these bases is the information.

A gene is a stretch of DNA that contains the information for making a particular protein (or a functional RNA). A chromosome is a structure in which long DNA is wound around proteins and packed together; chromosomes show up clearly as rod shapes when a cell divides. As an analogy, DNA is a long thread with writing on it, a gene is one paragraph of that writing, and a chromosome is the spool the thread is neatly wound on.

Human body cells contain 46 chromosomes, that is, 23 pairs. One of each pair comes from the mother and the other from the father. Of these, 22 pairs are autosomes, shared by males and females, and 1 pair is the sex chromosomes (XX in females, XY in males).

3.How DNA information becomes protein

DNA is kept inside the nucleus, but proteins are made by ribosomes outside the nucleus. So first, the needed part of a gene is copied into RNA, which is sent out of the nucleus. This step is called transcription. The ribosome reads the RNA's bases three at a time (each group of three bases is called a codon) and links the matching amino acids together one after another to build a protein. This step is called translation.

Proteins are building materials for muscle and hair, and they are also the workers that control the body's chemical reactions and signals, such as digestive enzymes, hormones, and antibodies. Genes affect traits mostly by way of these proteins.

The real process is far more complex. There are mechanisms that control which genes are used, when, and how much, and environment and lifestyle also affect how much genes are used. Saying “your genes determine your destiny” is an oversimplification for most traits.

4.Dominant and recessive — Mendel's principles of heredity

Through crossbreeding experiments with peas, Mendel showed that traits are passed on by a pair of hereditary factors (what we now call alleles). A trait that shows up when even one of the two factors is present is called dominant, and a trait that shows up only when both are present is called recessive. Dominant factors are usually written with a capital letter (A) and recessive ones with a lowercase letter (a).

Dominant does not mean “better” or “more common.” It simply means the one that shows on the outside when both factors are present. Also, most human traits, such as height, skin color, and eye color, are influenced by many genes together with the environment, so they can't be explained by a simple dominant-recessive rule. Even tongue rolling, which was often used as a textbook example, has been pointed out as not actually being neatly explained by a single pair of genes.

5.When there are three alleles — ABO blood types

In Mendel's pea example there were only two alleles, R and r, but some genes have three or more alleles. Human ABO blood types are the classic example. There are three alleles, A, B, and O, and each person gets two of them, one from each parent.

A and B are dominant over O, and when A and B are present together, neither hides the other; both show up, giving type AB. So genotypes AA and AO are type A, BB and BO are type B, AB is type AB, and only OO is type O. The important point here is that you can't pin down the genotype from the blood type (phenotype) alone. A person with type A could be AA or AO.

The ABO blood type is a rare example that a single gene explains fairly neatly. As we saw above, most traits are not this simple. And the idea that you can judge personality by blood type is a popular belief with no confirmed scientific basis.

6.Cell division — mitosis and meiosis

Your body grows and wounds heal thanks to mitosis (body-cell division). The chromosomes are copied and then split in two, so one cell becomes 2 cells with the same genetic information. In humans, there are 46 chromosomes both before and after division.

Making reproductive cells (sperm and eggs) involves meiosis. The cell divides twice in a row to produce 4 cells, and the number of chromosomes is halved to 23. When a sperm (23) fertilizes an egg (23), the number returns to 46, so the chromosome number is maintained from generation to generation. Because the chromosomes received from the parents are shuffled and distributed during meiosis, even siblings have different combinations of genes.