GENETICS (Part 3 of 3)

Sex Linkage

Linked genes occur on the sex chromosomes as well as on the non sex chromosomes, or autosomes. In humans, a woman carries two X chromosomes and 44 autosomes in each body cell and one X chromosome and 22 autosomes in each egg. A man carries one X and one Y chromosome and 44 autosomes in each body cell and either an X or a Y chromosome and 22 autosomes in each sperm cell.

Only sons inherit traits carried by genes located on the Y chromosome, because a boy (XY) develops whenever a Y sperm fertilizes an egg. Traits carried on genes located on an X chromosome of the father are transmitted only to daughters (XX).

GENES AND THE GENETIC CODE

Genes, the arbiters of body form and organ function, work with precision. They transmit to each cell a genetic code that determines the cell's purpose.

Nucleic Acids The Key to Heredity

Nucleic acid, any of substances comprising genetic material of living cells; divided into two classes: RNA (ribonucleic acid) and DNA (deoxyribonucleic acid); directs protein synthesis and is vehicle for transmission of genetic information from parent to offspring.

The structure of DNA makes gene transmission possible. Since genes are segments of DNA, DNA must be able to make exact copies of itself to enable the next generation of cells to receive the same genes.

Adenine, a purine base that codes hereditary information in the genetic code in DNA and RNA.

Cytosine, pyrimidine base that codes genetic information in DNA or RNA.

The DNA molecule looks like a twisted ladder. Each "side" is a chain of alternating phosphate and deoxyribose sugar molecules. The "steps" are formed by bonded pairs of purine-pyrimidine bases. DNA contains four such bases the purines adenine (A) and guanine (G) and the pyrimidines cytosine (C) and thymine (T).

The RNA molecule, markedly similar to DNA, usually consists of a single chain. The RNA chain contains ribose sugars instead of deoxyribose. In RNA, the pyrimidine uracil (U) replaces the thymine of DNA.

DNA and RNA are made up of basic units called nucleotides. In DNA, each of these is composed of a phosphate, a deoxyribose sugar, and either A, T, G, or C. RNA nucleotides consist of a phosphate, a ribose sugar, and either A, U, G, or C.

Nucleotide chains in DNA wind around one another to form a complete twist, or gyre, every ten nucleotides along the molecule. The two chains are held fast by hydrogen bonds linking A to T and C to G A always pairs with T (or with U in RNA); C always pairs with G. Sequences of the paired bases are the foundation of the genetic code. Thus, a portion of a double-stranded DNA molecule might read: A-T C-G G-C T-A G-C C-G A-T. When "unzipped," the left strand would read: ACGTGCA; the right strand: TGCACGT.

DNA is the "master molecule" of the cell. It directs the synthesis of RNA. When RNA is being transcribed, or copied, from an unzipped segment of DNA, RNA nucleotides temporarily pair their bases with those of the DNA strand. In the preceding example, the left hand portion of DNA would transcribe a strand of RNA with the base sequence: UGCACGU.

Genes and Protein Synthesis

A genetic code guides the assembly of proteins. The code ensures that each protein is built from the proper sequence of amino acids.

Genes transmit their protein-building instructions by transcribing a special type of RNA called messenger RNA (mRNA). This leaves the cell nucleus and moves to structures in the cytoplasm called ribosomes, where protein synthesis takes place.

Cell biologists believe that DNA also builds a type of RNA called transfer RNA (tRNA), which floats freely through the cell cytoplasm. Each tRNA molecule links with a specific amino acid. When needed for protein synthesis, the amino acids are borne by tRNA to a ribosome.

For years biologists wondered how amino acids were guided to fit together in the exact sequences needed to produce the thousands of kinds of proteins required to sustain life.

The answer seems to lie in the way the four genetic "code letters" A, T, C, and G are arranged along the DNA molecule.

The Genetic Code

Experimental evidence indicates that the genetic code is a "triplet" code; that is, each series of three nucleotides along the DNA molecule orders where a particular amino acid should be placed in a growing protein molecule. Three-nucleotide units on an mRNA strand for example UUU, UUG, and GUU are called codons. The codons, transcribed from DNA, are strung out in a sequence to form mRNA.

According to the triplet theory, tRNA contains anti codons, nucleotide triplets that pair their bases with mRNA codons. Thus, AAA is the anti codon for UUU. When a codon specifies a particular amino acid during protein synthesis, the tRNA molecule with the anti codon delivers the needed amino acid to the bonding site on the ribosome.

The genetic code consists of 64 codons. However, since these codons order only some 20 amino acids, most, if not all, of the amino acids can be ordered by more than one of them. For example, the mRNA codons UGU and UGC both order cysteine. Because mRNA is a reverse copy of DNA the genetic code for cysteine is ACA or ACG. Some codons may act only to signal a halt to protein synthesis.

To illustrate the operation of the genetic code, assume that one protein is responsible for the development of brown hair and that this protein is composed of three amino acid molecules arranged in linear sequence for example, cysteine-cysteine-cysteine. (This is a much simplified example, since proteins actually incorporate from 100 to 300 amino acid molecules.) The gene (DNA segment) specifying formation of this protein reads: ACAACAACA. It produces the mRNA segment UGUUGUUGU. This segment then drifts to a ribosome. Three tRNA molecules, each with the cysteine-bearing anti codon ACA, line up in order on the ribosome and deposit their cysteine to make the brown-hair protein.

Since code transmission from DNA to mRNA is extremely precise, any error in the code affects protein synthesis. If the error is serious enough, it eventually affects some body trait or feature.

Mutations

Down's syndrome (or mongolism), a congenital condition with moderate to severe mental retardation; characteristic features include: broad flat faces, slanted eyes, small ears and noses; heart defects and other abnormalities.

Certain chemicals and types of radiation can cause mutations changes in the structure of genes or chromosomes. The simplest type of mutation is a change in the DNA or RNA nucleotide sequence. Mutations may also involve the number of chromosomes or the gain, loss, or rearrangement of chromosome segments. If a mutation occurs in parental sex cells, the change is passed on to the offspring. In humans, an extra chromosome in body cells (47 instead of 46) has been implicated in Down's syndrome, a serious mental abnormality.

Most mutations are considered harmful and are, therefore, eventually eliminated. Some, however, enable an organism to adapt to a changing environment. Biologists believe that mutations have caused the many genetic changes involved in the evolution of species.

Assisted by Val W. Woodward

Genetic Terms

allele. One of the members of a gene pair, each of which is found on chromosomes; the pair of alleles determines a specific trait.

chromosome. A structure in the cell nucleus containing genes.

dominance. The expression of one member of an allelic pair at the expense of the other in the phenotypes of heterozygotes.

gene. One of the chromosomal units that transmit specific hereditary traits; a segment of the self-reproducing molecule, deoxyribonucleic acid.

genotype. The genetic make-up of an organism, which may include genes for the traits that do not show up in the phenotype.

heterozygous. Containing dissimilar alleles.

homozygous. Containing a pair of identical alleles.

phenotype. The visible characteristics of an organism (for example, height and colouration).

recessiveness. The masking of one member of an allelic pair by the other in the phenotypes of heterozygotes.