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

The Chromosome: Discovery, Number and Structure.

A human body cell holds 46 chromosomes, a fruit fly cell only 8, and a roundworm cell just 2. The count works almost like a species's ID card. In this note you will see what a chromosome is, who found it and when, how many each organism carries, and what every part inside one is called.

1. What a chromosome is, and who named it

Inside the nucleus sits a thread-like body. It can copy itself, it takes up stain, and it is built of nucleoprotein. It acts as hereditary material and also plays a part in mutation and variation. This is the chromosome.

A prokaryotic cell has no well-formed chromosome. It still holds DNA as the main component (RNA in some viruses), and this is called a prochromosome.

The history is short. Karl Nageli first saw chromosomes in a plant cell nucleus in 1842. In 1875 Strasburger noticed thread-like structures while a cell was dividing.

Both names arrived in the same year, 1888. Walter Flemming called the thread-like structures chromatin, and W. Waldeyer called them chromosomes. In Greek, Chroma is colour and soma is body, so chromosome means “coloured body”. It takes up basic stains.

In 1902 Sutton and Boveri said that chromosomes carry the hereditary traits. In 1921 Theophilus Painter first reported the human chromosome number. Tap the six events and pair each year with its name. Heitz, in 1928, comes back in section 3.

2. How many, and how big

The chromosome number runs from 2 to 1600 depending on the species. In higher organisms a body cell usually has between 2 and 80.

See the book's numbers as bars first. The red line is 80, the usual top for a body cell of a higher organism.

Keep the extremes in mind. Among ferns the highest is Ophioglossum reticulatum, with 1200. Among flowering plants the lowest is Haplopappus gracilis (2n = 4) and the highest Poa littarosa (2n = 506-530). Among animals the lowest is 2, in the roundworm Ascaris megalocephalus sub. sp. univalens. The highest is 2n = 1600, in the radiolarian Aulacantha sp.

Now the numbers from the book's table. Plants first: rice (Oryza sativa) 24, wheat (Triticum aestivum) 42, maize (Zea mays) 20, onion (Allium cepa) 16, cucumber (Cucumis sativus) 14, potato (Solanum tuberosum) 48.

More plants: tomato (Lycopersicon esculentum) 24, tobacco (Nicotiana tabacum) 28, papaya (Carica papaya) 18, cabbage (Brassica oleracea) 18, jute (Corchorus capsularis) 14, radish (Raphanus sativus) 18, groundnut (Arachis hypogaea) 40.

Animals: human (Homo sapiens) 46, cattle (Bos indicus) 60, goat (Capra hircus) 60, pigeon (Columba livia) 80, frog (Rana pipiens) 26, rabbit (Oryctolagus cuniculus) 44.

More animals: gorilla (Gorilla gorilla) 48, guinea pig (Cavia porcellus) 64, housefly (Musca domestica) 12, fruit fly (Drosophila melanogaster) 8, Culex mosquito (Culex pipiens) 6, roundworm (Ascaris megalocephalus) 2. Say the number in your head first, then tap the organism to check.

In size, a chromosome is usually 0.5-30 μm long and 0.2-2.0 μm across. The average length is 4-6 μm in humans, 3 μm in Drosophila and 8-12 μm in maize. Chromosomes are seen best at the metaphase of cell division, when they are well formed and separate.

3. Chromatin and how it coils

The main material of a chromosome is chromatin, a DNA-protein compound. DNA bound to histone protein is called a nucleosome.

Zoom out step by step and watch how DNA fits in so small a space: each level is a thicker coil.

The nucleoprotein thread starts 11 nm thick. It coils to 30 nm, then 300 nm, and finally 700 nm chromatin. In a human chromosome this shortens the DNA about 10,000 times. Put the six levels of Figure 1.18 in order from small to large.

In 1928 Heitz divided chromatin into two kinds. Heterochromatin is tightly coiled, inactive in heredity, and takes no part in mRNA synthesis. Euchromatin is loosely coiled, active, and makes mRNA. The tighter the coil, the quieter the chromatin.

4. The parts of a chromosome

At metaphase a chromosome lies as two parallel halves, split lengthwise. Each half is a chromatid, or sister chromatid. In the modern view one chromatid is one DNA molecule. Vejdovsky (1921) called it the chromonema.

In one unstained region the two sister chromatids are held tightly together. That is the centromere, and the notch it makes is the primary constriction. A typical chromosome has one centromere. In unusual cases it can have two or more, or none at all.

The parts on either side of the centromere are the arms. Every chromosome has two, equal or unequal. At the centromere sits a small structure, the kinetochore, and this is where microtubules attach.

At the start of meiotic prophase, tiny beads show along the chromosome. These are chromomeres. They are clearest at the pachytene substage of meiosis.

A constriction on an arm other than the primary one is a secondary constriction, also called the nucleolus organiser region. The nearly round part joined by a chromatin thread at the end of an arm is the satellite. A chromosome with a satellite is a SAT chromosome, and it carries the nucleolus. Cotton, jute and gram have such chromosomes.

By H. J. Muller, the special regions at both ends of a chromosome are the telomeres. The enzyme telomerase is thought to slow human ageing.

Around the chromatin thread, under the pellicle, is supposed to lie a layer of protein and RNA, the matrix. The thin imagined covering outside the matrix is the pellicle. Modern research has not proved that either exists. Now tap the nine points of Figure 1.19.

5. Check yourself

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

The second tests a fact from the book.

In the last, read every option closely.

The whole note on eight cards.

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