Cell Division and Heredity

Every living thing — from a tiny bacterium to a giant blue whale — is made of cells, the smallest units of life. But cells do not last forever, and a growing body constantly needs more of them. The process by which a single cell splits to form new cells is called cell division. Through cell division, one parent cell gives rise to one or more daughter cells. Living organisms divide their cells for three main reasons: growth, repair (and replacement), and reproduction.

The first reason is growth. A baby grows into an adult, and a seed grows into a tall plant, mainly because the number of cells increases, not because each cell becomes huge. In fact, a single fertilised egg (the zygote) divides again and again to produce the trillions of cells that make up a full human body. Cells stay small and divide because a small cell can take in food and oxygen and remove wastes far more efficiently than a very large one.

The second reason is repair and replacement. Throughout life, cells become old, worn out, damaged, or die, and they must be replaced by new cells made through division. When you get a cut on your skin, nearby cells divide to heal the wound and close the gap. Your body constantly replaces dead skin cells, old blood cells, and the lining of your gut — all through cell division. Without this steady replacement, even small injuries would never heal.

The third reason is reproduction — the making of new individuals. In single-celled (unicellular) organisms such as Amoeba and bacteria, cell division is reproduction: the parent simply divides into two new organisms. In multicellular organisms, a special kind of cell division produces gametes (sex cells like sperm and eggs) that join to form new offspring. So cell division lies behind growth, healing, and the continuation of life itself — and the orderly way cells carry it out is studied as the cell cycle.

A cell does not divide the moment it is born. Instead, every cell passes through an orderly, repeating sequence of stages called the cell cycle — the complete life of a cell from the time it is formed until it divides into two. The cell cycle makes sure that a cell grows, copies its contents accurately, and only then divides, so that each new daughter cell receives everything it needs. The cell cycle has two main parts: a long resting-and-growing part called interphase, and a shorter dividing part called the mitotic (M) phase.

Interphase is the longest part of the cell cycle, during which the cell is busy but not dividing. Although it was once wrongly thought to be a "resting" stage, interphase is actually a period of great activity — the cell grows, carries out its normal work, and prepares for division. Interphase is divided into three sub-phases: G1, S, and G2. In the G1 phase (first gap/growth phase), the cell grows larger, makes new proteins, and increases the number of its organelles. In the S phase (synthesis phase), the cell copies (replicates) its DNA, so that there is now a complete double set of genetic information. In the G2 phase (second gap phase), the cell grows a little more and checks the copied DNA, getting ready to divide.

After interphase comes the mitotic phase (M phase), the part where actual division happens. It includes mitosis — the division of the nucleus, in which the copied chromosomes are shared equally — followed by cytokinesis, the division of the cytoplasm, which splits the cell into two separate daughter cells. Because the DNA was carefully doubled during the S phase, each daughter cell ends up with a full, identical set of genetic information.

The cell cycle is tightly controlled by the cell, with checkpoints that make sure each step is completed correctly before the next begins. This control matters: if a cell divides without copying its DNA properly, the daughter cells would be faulty. When this control breaks down and cells divide uncontrollably, it can lead to diseases such as cancer. So the cell cycle — interphase (G1, S, G2) followed by the mitotic phase — is the well-ordered programme that allows cells to grow and divide safely and accurately.

Mitosis is the type of cell division in which a parent cell divides to produce two daughter cells that are exactly identical to it and to each other. It is the division of the nucleus in which the copied chromosomes are shared equally. Mitosis takes place in the body (somatic) cells and is responsible for growth, repair, and the replacement of worn-out cells. Although mitosis is one continuous process, scientists describe it in four main stages for convenience: prophase, metaphase, anaphase, and telophase.

The first stage is prophase. During prophase, the long thread-like chromosomes (already copied in the S phase) coil up and become short, thick, and visible, each made of two identical halves called sister chromatids joined at a point. The nuclear membrane begins to break down, and thin fibres called spindle fibres start to form. The second stage is metaphase, in which the chromosomes line up neatly along the middle (equator) of the cell, attached to the spindle fibres. This lining-up makes sure that the chromosomes can be shared exactly equally.

The third stage is anaphase, the stage of separation. Here the sister chromatids are pulled apart by the spindle fibres and move to opposite ends (poles) of the cell, so that each pole receives one full set of chromosomes. The fourth stage is telophase, in which the chromosomes reach the two poles, uncoil back into thin threads, and a new nuclear membrane forms around each set. Now there are two nuclei inside one cell. Telophase is usually followed by cytokinesis, the splitting of the cytoplasm, which finally produces two separate daughter cells.

The significance of mitosis is great. Because the DNA was copied beforehand and then shared equally, each daughter cell has exactly the same number and kind of chromosomes as the parent cell — they are genetically identical. This allows an organism to grow while keeping all its cells the same, to repair injuries, and to replace old cells without changing its genetic make-up. So mitosis can be summed up as: one parent cell → (prophase, metaphase, anaphase, telophase) → two identical daughter cells.

While mitosis makes identical body cells, a different kind of division is needed to make gametes — the sex cells such as sperm and eggs. This special division is called meiosis. In meiosis, a parent cell divides twice but the DNA is copied only once, so the result is four daughter cells, each with half the number of chromosomes of the parent cell. Cells with a full (double) set of chromosomes are called diploid, and cells with half the set are called haploid. So meiosis takes one diploid cell and produces four haploid gametes.

The reason for halving the chromosome number is simple but vital. During reproduction, a sperm and an egg join together in fertilisation. If both gametes had the full chromosome number, their union would double the number in the offspring, and it would keep doubling in every generation. By making gametes haploid, meiosis ensures that when sperm and egg combine, the offspring gets back the correct, normal (diploid) chromosome number — half from the mother and half from the father. In humans, each gamete carries 23 chromosomes, and fertilisation restores the full 46.

Meiosis happens in two rounds of division called meiosis I and meiosis II. In meiosis I, similar (matching) chromosomes from the mother and father, called homologous chromosomes, pair up and then separate into two cells, halving the chromosome number. A key event here is crossing over (recombination), in which paired chromosomes exchange pieces of genetic material. In meiosis II, which is similar to mitosis, the chromatids separate, finally producing four haploid cells in total.

The significance of meiosis is twofold. First, it forms gametes and keeps the chromosome number constant from generation to generation. Second — and very importantly — it creates genetic variation. Through crossing over and the random way chromosomes are shuffled into gametes, every gamete carries a slightly different combination of genes. This is why brothers and sisters are not identical (except identical twins) and why offspring differ from their parents. So meiosis can be summed up as: one diploid cell → two divisions → four haploid, genetically varied gametes.

Inside the nucleus of every cell lie the instructions that control how a living thing is built and how it works. These instructions are stored in thread-like structures called chromosomes. A chromosome is a tightly coiled package of a very long molecule called DNA wound around proteins. When a cell is not dividing, the DNA is spread out as fine threads, but as the cell prepares to divide it coils up into the short, thick chromosomes we can see under a microscope. Each chromosome usually appears as an X-shape after the DNA has been copied, made of two identical halves joined at a point called the centromere.

The molecule that makes up chromosomes is DNA (deoxyribonucleic acid). DNA is shaped like a twisted ladder, the famous double helix. The "rungs" of this ladder are pairs of chemical units called bases, and the exact order (sequence) of these bases acts like a code — a set of instructions written in a chemical language. This code tells the cell how to make proteins, which carry out and control almost everything in the body. DNA can also be copied accurately, which is what allows it to be passed on during cell division and inheritance.

A short section of DNA that carries the instructions for a particular feature or protein is called a gene. Genes are the basic units of heredity — the units passed from parents to offspring. One gene might carry instructions for eye colour, another for the ability to roll the tongue, and so on. A single chromosome carries many genes arranged along its length, like beads on a string. Because offspring inherit chromosomes (and so the genes on them) from their parents, they inherit their parents' features.

Different living things have different numbers of chromosomes, and the number is fixed for each species. Human body cells contain 46 chromosomes, arranged in 23 pairs — one of each pair inherited from the mother and one from the father. Of these 23 pairs, 22 pairs are ordinary chromosomes called autosomes, and 1 pair are the sex chromosomes that decide whether a person is male or female. So the relationship to remember is: chromosomes are made of DNA; DNA carries genes; and genes are the units of heredity — and in humans there are 46 chromosomes in every body cell.

Heredity is the passing on of features (characters) from parents to their offspring. It is the reason children resemble their parents in traits such as eye colour, height, or the shape of the nose. The scientific study of heredity and the rules by which traits are inherited is called genetics. The foundation of genetics was laid by an Austrian monk named Gregor Johann Mendel, who is honoured today as the "Father of Genetics." Mendel carried out careful experiments on the ordinary garden pea plant in the 1860s and discovered the basic laws of inheritance.

Mendel chose the pea plant for good reasons: it grows quickly, produces many offspring, and shows clear, contrasting features that are easy to observe — for example, tall vs short plants, round vs wrinkled seeds, and green vs yellow pods. He could also control which plants bred with which by cross-pollinating them by hand. By studying one pair of contrasting traits at a time and counting the offspring over several generations, Mendel turned breeding into a careful, mathematical study — something no one had done so clearly before.

In a typical experiment, Mendel crossed a pure tall pea plant with a pure short one (the parent or P generation). Surprisingly, all the offspring (the first filial or F1 generation) were tall — the shortness seemed to disappear. Mendel then let these F1 tall plants self-pollinate. In the next generation (the F2 generation), the short plants reappeared, and the tall and short plants appeared in a fixed ratio of about 3 tall : 1 short. This showed that the factor for shortness had not been lost in the F1 plants — it had only been hidden.

From such results Mendel proposed that each trait is controlled by a pair of "factors" (which we now call genes), one inherited from each parent. He gave two great rules. The Law of Segregation states that the two factors for a trait separate during the formation of gametes, so each gamete carries only one factor of the pair. The Law of Independent Assortment states that the factors for different traits are passed on independently of one another. These laws explain how traits hide and reappear and form the basis of all modern genetics.

Mendel's experiments showed that when two contrasting factors come together, one trait often shows up while the other stays hidden. The trait that appears (expresses itself) even when only one factor for it is present is called the dominant trait. The trait that stays hidden when the dominant factor is present, and shows up only when no dominant factor is there, is called the recessive trait. In pea plants, tallness is dominant over shortness, which is why the F1 plants were all tall even though they carried a hidden factor for shortness.

To describe inheritance clearly, scientists use symbols: a capital letter for the dominant factor (allele) and the same letter in small for the recessive one. For example, T stands for tall (dominant) and t for short (recessive). Since each plant has a pair of factors for a trait, a plant can carry the combinations TT, Tt, or tt. These different forms of a gene (like T and t) are called alleles. The combination of alleles an organism carries for a trait is its genotype.

The genotype (the genetic make-up, such as TT, Tt, or tt) decides the phenotype, which is the visible feature or appearance of the organism (such as being tall or short). A plant with genotype TT or Tt will look tall (phenotype tall), because the dominant T is present. Only a plant with genotype tt will be short, because there is no dominant factor to hide the recessive one. This is why the same appearance (tall) can come from two different genotypes (TT and Tt).

When the two alleles in a pair are the same, the organism is said to be homozygous for that trait — for example TT (homozygous dominant) or tt (homozygous recessive). When the two alleles are different, the organism is heterozygous — for example Tt. A heterozygous (Tt) plant looks tall but carries a hidden recessive factor, so it can pass shortness to its offspring. Understanding dominant and recessive traits, genotype and phenotype, and homozygous and heterozygous conditions explains exactly how traits hide in one generation and reappear in the next.

Among the 23 pairs of chromosomes in human cells, 22 pairs are ordinary chromosomes called autosomes, and 1 special pair decides whether a person is male or female. This special pair is called the sex chromosomes, and there are two kinds: the X chromosome and the Y chromosome. A female has two X chromosomes — her sex-chromosome pair is XX. A male has one X and one Y chromosome — his pair is XY. So the simple rule is: XX = female, XY = male.

How is the sex of a baby decided? It depends on the chromosome carried by the sperm, not the egg. Because the mother is XX, every egg she makes carries an X chromosome. The father is XY, so half of his sperm carry an X and half carry a Y. If a sperm carrying X fertilises the egg, the baby is XX (a girl). If a sperm carrying Y fertilises the egg, the baby is XY (a boy). This gives an equal 50 : 50 chance of a boy or a girl, and shows that the father's sperm determines the sex of the child.

The X and Y chromosomes do more than decide sex — they also carry genes for certain other traits. Traits whose genes are located on the sex chromosomes (usually the X chromosome) are called sex-linked traits. Because males have only one X chromosome, a single recessive allele on it will show up in them, with no second X to hide it. Females, with two X chromosomes, are more likely to carry such a recessive allele hidden on one X. This is why several sex-linked conditions appear more often in males than in females.

Two well-known sex-linked traits in humans are colour blindness and haemophilia. Colour blindness is the inability to tell certain colours apart, most often red and green; its gene is recessive and carried on the X chromosome, so it is far more common in men. Haemophilia is a condition in which the blood does not clot properly, so even small injuries can cause prolonged bleeding; it too is a recessive X-linked condition seen mostly in males. A woman who carries the allele on one X but does not show the condition is called a carrier, and she can pass it on to her sons. So sex determination (XX and XY) also explains the inheritance of these sex-linked traits.

Usually, DNA is copied very accurately and passed on faithfully from cell to cell and parent to offspring. But sometimes a sudden, permanent change occurs in the DNA or chromosomes of an organism. This change is called a mutation. Because genes carry the instructions for an organism's features, a mutation can change those instructions and so change a trait. Mutations are the ultimate source of new variation in living things, but many of them are harmful, some are neutral (no effect), and a few may even be helpful. Mutations are broadly of two kinds: gene mutations and chromosomal mutations.

A gene mutation is a change in the DNA of a single gene — for example, a change in the sequence of bases that make up the gene. Even a small change can alter the protein the gene makes and so change a trait. A chromosomal mutation is a larger change involving whole chromosomes — for example, a change in their structure (a piece breaking off or rearranging) or in their number (a cell ending up with one chromosome too many or too few). Chromosomal mutations in number often happen when chromosomes fail to separate properly during meiosis.

A well-known example caused by a change in chromosome number is Down syndrome. It occurs when a person has an extra copy of chromosome number 21 — three copies instead of the usual two — so the body cells have 47 chromosomes instead of 46. Down syndrome leads to certain physical features and some degree of learning difficulty. It is usually caused by the failure of chromosome 21 to separate correctly during the formation of a gamete.

A famous example caused by a gene mutation is sickle cell anaemia. Here a tiny change in the gene for haemoglobin (the substance in red blood cells that carries oxygen) makes the red blood cells take on a curved, sickle (crescent) shape instead of their normal round shape. These misshapen cells carry less oxygen and can block blood vessels, causing illness. Mutations can be caused by several factors, including certain chemicals, radiation (such as X-rays and ultraviolet rays), and natural mistakes during DNA copying. So a mutation is a permanent change in the genetic material — gene or chromosomal — and it can lead to conditions such as Down syndrome and sickle cell anaemia.