Biology 1st Paper (Botany) · Classes 11-12 · Chapter 1 · 1.4

Gene and Genome.

What decides the colour of your eyes, or of a flower? Chasing that question, scientists reached a unit called the gene. This note covers what a gene is, who found it, what is inside it, and what all the genes together make up.

1. What a gene is, and where the name came from

The smallest unit that controls a trait of an organism is called a gene.

In the 1860s Mendel spoke of a particle, or factor, that carries a trait. In 1909 Johannsen was the first to call it a gene. In 1912 T. H. Morgan showed that genes sit on chromosomes, and the place on a chromosome where one gene sits is its locus. Later Har Gobinda Khorana synthesised an artificial gene and won the Nobel Prize in 1968.

Walk along the years on the line. Winkler's 1920 comes back in the genome section.

2. One gene one enzyme, and the gene is DNA

In 1941 Beadle and Tatum worked on the fungus Neurospora crassa. They showed that one specific gene is responsible for making one specific enzyme.

This gave the 'one gene one enzyme' theory. Enzymes are proteins, so the theory was later refined to 'one gene one polypeptide chain'. Different arrangements of amino acids make many different enzymes, which is why protein is called the language of life.

The answer to what a gene is made of came from work on Pneumococci. Frederick Griffith found that the virulent form's trait of making a capsule can be passed from one cell to another. O. T. Avery showed that DNA carries it across. So DNA is the gene.

3. The four units of the gene

In the modern view a gene is expressed in four units: recon, muton, replicon and cistron. Each has its own job.

The recon is the unit of gene recombination: the smallest part of DNA that takes part in recombination. It holds one or two nucleotide pairs. The muton is the unit of gene mutation: the smallest part of DNA that can mutate. It holds one or more nucleotide pairs. Mind the difference: “one or two” for the recon, “one or more” for the muton.

The replicon is the part of DNA that controls the replication of DNA. The cistron is the unit of gene function: the DNA segment that controls the activity of cellular material. One cistron of E. coli has about 1500 nucleotide pairs.

Every cistron holds many recons and mutons, so it is far longer than either. Usually a gene and a cistron are nearly equivalent. Now read each description and pick the unit.

4. What a gene is made of

In structural terms a gene is a specific DNA sequence at a locus of a chromosome, carrying the message for building one working polypeptide chain.

Genes are made of nucleic acid. In eukaryotic cells they lie on chromosomes; in prokaryotic cells they lie in the nuclear material or in plasmids.

Genes play the key role in variety and mutation, and they carry traits from one generation to the next. A single trait may need more than one gene.

5. The genome

The sum of all the genes on one set of chromosomes of a species is its genome. The German botanist Hans Winkler first used the word genome in 1920.

The human genome has 3000 million base pairs, spread over 24 chromosomes (22A + 1X + 1Y). The genome of all humans is 99.9 percent the same.

Of this huge genome only 2 percent, the genes, takes part in expressing traits. The other 98 percent is inactive and called junk DNA. The human genome is 98 percent like the chimpanzee's and 97 percent like the gorilla's.

6. Mutation, exons and introns

A mutation in any gene can give a permanent new variant that is passed down the generations. And some traits, such as height, are controlled by several genes together.

What does changing one base do? Make a mutation yourself and read it with the book's codon table.

Not all of a eukaryotic gene carries instructions for making protein. The coding parts are exons and the non-coding parts are introns. Only exons take part in protein synthesis. A gene with introns lying between its exons is a split gene, seen in mammals, reptiles and birds.

Genes differ hugely in number and in size. A mammalian cell may have more than 50,000 genes. The smallest recorded gene has 75 nucleotides and the largest has 80,000.

7. Check yourself

Three questions in the exam's style. Start with the first.

Now the second.

The last one, read it slowly.

The whole note on eight cards.

Source: Biology 1st Paper (Botany), Classes 11-12 (NCTB), Chapter 1 “Cell and Its Structure”, section 1.4.