" The Big Picture!" by Mr C

Molecular Level of Genetics
Most of the molecules found in humans and other living organisms fall into one of four categories:

1. carbohydrates (sugars and starches) 2. lipids (fats, oils, and waxes)
3. proteins
4. nucleic acids

Proteins are large chain-like molecules that are twisted and folded back on themselves in complex patterns. They serve as structural material for the body, gas transporters, hormones , antibodies , neurotransmitters , and enzymes . In fact, when looking at someone, you mostly see proteins since skin and hair are primarily made of them. Proteins acting as enzymes are particularly important substances because they trigger and control the chemical reactions by which carbohydrates, lipids, and other substances are created. When you look at another human being, you mostly see proteins.

Our bodies produce about 90,000 thousand different kinds of proteins, all of which consist of sim- pler units called amino acids .

Proteins in all organisms are mostly composed of just 20 kinds of amino acids. Proteins differ in the number, sequence, and kinds of amino acids. Our bodies produce some of these amino acids, while others come directly from food that we consume.

AMINO ACIDS

Proteins, and subsequently amino acids, are mostly made up of just four elements: carbon, oxygen, hydrogen, and nitrogen. In fact, 96.3% of your body is composed of these common elements.

The largest molecules in people and other organisms are nucleic acids. Like proteins, they consist of very long chains of simpler units. However, the components, shapes, and functions of nucleic acids differ significantly from those of proteins. There are two basic varieties of nucleic acids: DNA (deoxyribonucleic acid ) and RNA (ribonucleic acid ). Both play critical roles in the produc- tion of proteins.

A chromosome consists mainly of one or more very long DNA molecules. Each of these molecules contains the genetic codes, or genes, for the synthesis of many different proteins and for the regula- tion of other genes. In a sense, a DNA molecule is a linear sequence of permanently stored blue- prints or recipes that are used regularly by our cells to make proteins out of amino acids.

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These new identical messenger RNA molecules then leave the nucleus and go out into the cytoplasm where the protein they are coded for is actually synthesized or assembled.

mRNA migrating out of the cell nucleus

Specifically, the messenger RNA molecules migrate from the chromosomes to the ribosomes, which are small graules in the cytoplasm. Some ribosome are on the surface of mem- brane networks called endoplasmic reticula , while others are free ribosomes. Assembly of proteins takes place at the site of the ribosomes

DNA copied

mRNA migrates to cytoplasm

Generalized animal cell

Protein synthesis begins as ribosomes move along the messenger RNA strand and attach transfer RNA (tRNA) anticodons (each with 3 bases) to triplets of complementary bases on the mRNA.

protein molecule forming

Protein synthesis at the ribosomes initiated by mRNA momentarily bonding with tRNA (Note: this schematic representation is a simplification of the actual process.)

Each transfer RNA attracts and brings a specific amino acid along with it. As a ribosome translates the messenger RNA code, a protein is assembled lineally, one amino acid at a

chromosomes ribosomes

(small dots)

endoplasmic reticula

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Not all codons specify amino acid components to be included in a protein. For instance, a start codon appears in DNA at thebeginning of the lineal code sequence for each gene and a stop codon is at the end. In other words, they indicate where a protein recipe begins and ends.

Most plant and animal cells have tens of thousands of ribosomes. Many ribosomes simultaneously translate identical strands of messenger RNA. As a result, the synthesis of proteins can be rapid and massive. These same processes can occur at the same time in millions of cells when a particu- lar protein is needed.

In addition to keeping the blueprints for protein synthesis, DNA has one further function–it replicates, or duplicates, itself. At the beginning of this process, the parent DNA molecule unwinds and unzips along its bases beginning at one end. Then in response to an enzyme, free nucleotides pair up with cor- responding bases on both of the DNA strands, as illustrated below. This results in the formation of two exact copies of the original molecule. Nuclear DNA replication occurs just before mitosis and meiosis.

DNA replication

Occasionally, an error is made in DNA replication. For example, an incorrect base pair may be included. This constitutes a mutation. If it occurs in the formation of sex cells, the mutation may be inherited and passed on in future generations. Such errors in replication are the ultimate sources of all new genes and are essential for the evolution of new species. They are also responsible for changes in somatic cells that result in the uncontrolled tumorgrowths of cancer.

It is important to realize that the genetic code system of humans is not unique but is shared by all living things. The same codons code for the same amino acids in people, dogs, fleas, and even bac- teria. In addition, we share many genes with other creatures. For instance, about 90% of human genes are identical to those of a mouse. Even more surprising is the fact that more than 1/3 of our genes are shared with a primitive group of worm species known as nematodes. The universal nature of the genetic code is compelling evidence for the evolution of all organisms from the same early life forms.

©1998, 2000 by Dennis O’Neil. Used by permission of the author.