10 Nature, structure and organisation of the genetic material

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Mendel’s model of inheritance 1. Each trait was controlled by a pair of inherited factors. For example, the trait ‘seed colour’ was assumed to be controlled by a pair of factors, with one producing ‘yellow’ and the other producing ‘green’. 2. For each trait, individual plants had two factors that could be identical or different. Plants with two identical factors (such as ‘long’ and ‘long’) were referred to as pure breeding, while plants with different factors (such as ‘long’ and ‘short’) were called hybrids. 3. Each factor was a discrete particle that retained its identity across generations. This idea challenged the commonly held view that inheritance was a blending process in which factors lost their identity (see figure 10.9). 4. The character that was expressed in the F1 hybrid plants was dominant, while the hidden character in the hybrid was recessive. For example, green pod colour is dominant and yellow pod colour is recessive. 5. During gamete formation, the members of each pair of factors separated to different gametes, with one factor per gamete. This is the principle of segregation of alleles or Mendel’s first law. 6. In separating, members of one pair of factors behaved independently of members of other pairs of factors. This is the principle of independent assortment or Mendel’s second law. 7. The results of a particular cross were the same, regardless of which plant was used as the male parent and which as the female parent.

Mendel carried out carefully planned experiments using techniques different from other plant breeders. Mendel developed a model of inheritance built on a set of assumptions about his factors. Mendel’s model both explained observed results and allowed predictions. Mendel’s model of inheritance was ignored by the scientific community but was rediscovered in 1900 independently by three biologists. After its rediscovery, Mendel’s model was soon found to apply to other kinds of living things.

Where are genes located?

Genes are located on chromosomes. The first suggestion that genes were located on chromosomes was made in 1902 by Sutton. In 1910, Morgan and his co-workers carried out the first experiments that demonstrated that genes were located on chromosomes. Griffith demonstrated the chemical nature of the genetic material by showing that a substance existed in bacteria that could change or transform one strain of harmless bacteria into a lethal strain and that this change was then passed on to the next generation. Avery’s experiments led to the conclusion that the transforming factor, and hence the genetic material, is DNA (deoxyribonucleic acid).

Analysing DNA The genetic material DNA is built of sub-units called nucleotides joined to form a linear chain. Each nucleotide consists of a deoxyribose sugar part, a phosphate part and an N-containing base. The sugar and phosphate parts are identical in nucleotides found in DNA, but there are four different bases, namely adenine (A), guanine (G), cytosine (C) and thymine (T). Chargaff’s rule states that certain bases occur in equal proportions in DNA.

DNA forms a double helix

DNA normally exists as a double helix molecule. In a DNA double helix, bases along one chain pair with complementary bases in the other chain and are stabilised by weak hydrogen bonds. Double-helical DNA can be reversibly dissociated into two single DNA chains by heating and the chains re-associate on cooling.

Relating DNA to chromosomes and genes The length of a double-helical DNA molecule can be expressed as the number of base pairs (bp) it contains. Each human chromosome contains one long molecule of double stranded DNA with millions of base pairs. A typical gene consists of tens of thousands of base pairs. The estimated total number of human genes is to

Of the two DNA chains in a gene, the one containing the genetic information is known as the template strand of DNA, while its complementary chain is called the non-template strand. Genetic instructions are coded in an ‘alphabet’ of four letters only: (the nucleotides) A, T, C and G. Identification of the order of nucleotides along a length of DNA is called DNA sequencing. Different genes vary in the nucleotide sequences along their DNA.

Nature of the genetic code DNA contains information encoded in the base sequence of its template strand. Genes contain coded instructions for joining specific amino acids into proteins. The genetic code in DNA is a non-overlapping triplet code consisting of groups of three bases. One piece of genetic code typically contains the information to add one amino acid to a protein.

What is a genome? The genome of an organism consists of its complete set of genetic instructions. The human genome consists of about 3000 million base pairs of DNA. The Human Genome Project was completed in Comparative genomics will provide new insights to our understanding of evolution.

Genetic material: stable or changing? The genetic material DNA is usually stable. DNA can undergo change (mutation). Mutations of DNA can vary and include deletions, substitutions and additions of nucleotides, as well as the type known as trinucleotide repeat expansions. Agents that can cause mutations are known as mutagenic agents. Mutations can be somatic or germline, and only germline mutations can be transmitted to the next generation. DNA mutations often, but not always, have deleterious results.

A closer look at a gene Each gene in eukaryote organisms contains a coding region, and also includes flanking regions upstream and downstream of the coding region. The coding region of a gene typically consists of several exons separated or interrupted by introns.

CODING AND FLANKING REGIONS promoters American traffic lights!