Nucleus and prokaryotic and eukaryotic cells

Biology · The cell. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The nucleus controls the cell because it holds the DNA. It is surrounded by the nuclear envelope, a double membrane with pores through which materials such as mRNA pass to the cytoplasm. Inside are the nucleolus and the chromatin, a network of DNA and protein. When a cell divides, the chromatin coils up into chromosomes.

Cells are of two kinds. Prokaryotic cells (bacteria and cyanobacteria) have no nuclear envelope and no membrane-bound organelles. Eukaryotic cells have a true nucleus and organelles such as mitochondria.

Plant and animal cells are both eukaryotic, but they differ in the cell wall, chloroplasts, vacuoles and centrioles.

Rules to remember

Nucleolus: no membrane; makes rRNA and assembles the subunits of ribosomes.
Euchromatin is loosely coiled, lightly stained and active; heterochromatin is tightly coiled, darkly stained and inactive.
A chromosome at the start of division has two chromatids joined at one centromere. Centromere in the middle = metacentric; slightly off the middle = sub-metacentric; near one end = acrocentric; at the end = telocentric.
Chromosome number is fixed for a species: human 46, chimpanzee 48, frog 26, fruit fly 8, onion 16, garden pea 14.
Prokaryotes: DNA in a nucleoid, circular and without histones; 70S ribosomes; wall of peptidoglycan (murein); divide by binary fission.
Eukaryotes: nuclear envelope; linear DNA with histones; 80S ribosomes in the cytoplasm; membrane-bound organelles; divide by mitosis or meiosis.
Plant cell: cellulose cell wall, chloroplasts, large central vacuole, no centrioles. Animal cell: no cell wall, no chloroplasts, small vacuoles, centrioles present.

Common mistake

Students write that prokaryotes have no DNA or no ribosomes. They have both: the DNA simply lies in a nucleoid without an envelope, and the ribosomes are the smaller 70S type.

Worked example

A human body cell has 46 chromosomes. At the start of mitosis each chromosome is made of two chromatids. How many chromatids and how many centromeres are in the nucleus?

Each chromosome has 2 chromatids: 46 × 2 = 92 chromatids.
The two chromatids of a chromosome are joined at one centromere.
So centromeres = number of chromosomes = 46.

Answer: 92 chromatids and 46 centromeres.

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Chapter 4: The Cell

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