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CHROMOSOMAL BASIS OF HEREDITY

Within the nucleus of non-dividing cells are strands or string-like structures known as chromosomes. As the cell is about to divide, the chromatids become more visible as strands with bead-like structures on them. These are the chromosomes and contain genes, which carry the genetic information which are transmitted from the parent to the offspring. We may then say that the chromosome is a nuclear component having a special organization, individuality and function. It is capable of self-reproduction (duplication) during cell division and of maintaining, its morphological and physiological properties through successive cell divisions.

Within the chromosomes and arranged in a linear order are the elements or factors that determine both the mental, physical and physiological attributes of the individual. These factors are called the genes. One can, therefore, say that a gene is a unit of heredity, that is, the element that determines the character of the individual such as height, or the stem length, colour of flower and so on. Studies have shown that these genes are made up of a long chain of deoxyribonucleic acid molecules (DNA).

 

DNA AND RNA

DNA (deoxyribonucleic acid) is a chemical substance present in the nucleus of every cell. It is passed from parents to offspring and is being regarded as a code of life. Since DNA is present in the nucleus and is acidic in nature, it is also referred to as nucleic acid.

 

The structure of DNA

DNA is made up of two chains of small chemical units called nucleotides. Each nucleotide consists of a phosphate group, a sugar called deoxyribose and a nitrogenous base. The nitrogenous bases are purines and pyrimidines. The purines are adenine (A) and guanine (G) while the pyrimidines are thymine (T) and cytosine (C).

The nucleotides are twisted into each other to form a double helix, something like a spiral staircase. The chemical structure of these bases are bond in certain pattern as adenine always joins thymine and guanine always joins cytosine. In other words the nitrogenous base from one chain (purine) links up with a compatible base (pyrimidine) of the opposite chain like rungs (steps) of a ladder. The four bases of the DNA chains are held together by hydrogen bonds.

 

The role of DNA

DNA transcribes the hereditary characteristics of the parents onto the offspring.

DNA keeps the number of chromosomes peculiar to the organism constant from generation to generation. DNA does this by doubling its number (replication or duplication) during mitosis and meiosis in the processes of asexual and sexual reproduction. Under the arrangement, the newly divided cells contain the complementary number of chromosomes of the parent.

DNA controls growth and development in every living thing. It does this by controlling protein and enzyme synthesis in the cytoplasm of the cell. The growth of an organism solely depends on the type of protein it can build following the instructions passed onto it by the DNA. At the same time the kind of enzymes an organism can secrete determines its development. The kind of enzymes secreted by an organism is directly controlled by the DNA. The DNA uses three types of ribonucleic acid (RNA); (messenger RNA, ribosomal RNA and transfer RNA) in starting and controlling the synthesis of proteins and enzymes in the cytoplasm of the cell since it does not leave the nucleus.

  1. It uses the messenger RNA (mRNA) to dispatch specific codes for the synthesis of protein to the cytoplasm.
  2. DNA controls the synthesis of ribosomal RNA (rRNA) and later dispatches it to the cyto plasm where it is responsible for the attachment of ribosomes to the messenger RNA.
  3. DNA dispatches transfer RNA (tRNA) into the cytoplasm where it picks and hands over various amino acids to the ribosomes to be joined together in long chains during protein synthesis.

Enzymes are protein in nature and are therefore synthesized under the same circumstances in the cytoplasm under the control of the DNA. All the differences among the living things are due to the number and type of codes contained in the DNA of the individual organism. For example most animals lack the cellulose digesting enzymes called cellulase. Such animals starve when placed on a diet that contains more cellulose than other food types. The absence of melanin pigment in the skin accounts for albinism. The albinos lack the enzyme tyrosinase and so cannot secrete tyrosine for the production of melanin.

 

The structure of RNA

The ribonucleic acid commonly called RNA is similar to the deoxyribonucleic acid, DNA. They are both referred to as nucleic acid because they are acidic and are found in the nucleus even though RNA is widely distributed in the cytoplasm of the cell.

 

RNA is different from DNA in having a sugar which contains oxygen while the sugar of DNA has no oxygen hence the name deoxyribonucleic acid. The nitrogenous bases purines are the same in both DNA and RNA while they differ in the pyrimidines. The pyrimidines of DNA consist of thymine and cytosine while those of RNA are made up of uracil and cytosine. Both DNA and RNA carry genetic code. This is why some viruses that lack DNA, have RNA to carry out their genetic functions.

RNA occurs as half of a ladder. It consists of a single upright strand of sugar and phosphate units. The nitrogenous bases of RNA are not in pairs, they occur singly. Three of the four nitrogenous bases are the same as those of DNA, adenine (A), cytosine (C), guanine (G). The fourth base thymine (T) is replaced with uracil (U) in RNA.

There are three types of RNA synthesized by the nucleus under the command of the DNA. These are:

1) Messenger RNA (mRNA)

This is the strand of RNA that collects the specific codes for the synthesis of particular amino acid in its nucleotide bases. It breaks off and moves through the nuclear membrane into the cytoplasm. Ribosomes get attached to the strand of the RNA and protein is synthesized.

 

2) Ribosomal RNA, (rRNA)

It is synthesized in the nucleus under the control of the DNA. It is responsible for the attachment of ribosomes to messenger RNA. It regulates the enzymes involved in the activities of ribosomes.

 

3) Transfer RNA, (tRNA)

The strands of transfer RNA are shorter than those of the messenger RNA. Various amino acids that will be joined together in long chains during protein synthesis are dispersed throughout the cytoplasm. There is a system in the messenger RNA that determines the exact amino acids and the sequence in which they are linked. The role of the transfer RNA is to pick the amino acids individually and bring them to the ribosomal RNA, in the proper order during the protein synthesis.

Some early studies by Drs. Morgan, Bridges and Sturtevant indicated that some of the hereditary characteristics of the fruit fly, Drosophila, could be related to definite regions for the chromosomes of this animal. By a careful analysis of the results from breeding flies with different features, it was possible to locate within the chromosomes the regions that control specific hereditary characteristics.

Studies carried out with the chromosomes in the salivary glands of the fruitfly (Drosophila) to determine their structure have produced very interesting and valuable results. The chromosomes in these glands are many times larger than those in the gametes or the other body (somatic) cells of the animal. Evidence shows that these large chromosomes are made up of numerous strands of chromatids, perhaps as many as a thousand. Such chromosomes are said to be polygenic. If these giant chromosomes are properly stained, alternating bands of dark and light materials can be seen. By comparing these giant chromosomes with ordinary chromosomes it has been found that bands are present at exactly the same points on the chromosomes at which previous methods forecast the presence of genes.

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