Heredity and Genetic Material

Nuclues, Chromosomes and Genes

Children often hear that they look like their mother or father. The reason is that offspring inherit genetic material from their parents. In earlier grades, you learned about heredity. Now we will see how this material is stored in cells.

Some cells have a nucleus – these are called eukaryotic cells. Others do not have a nucleus, and their genetic material floats freely in the cytoplasm – these are prokaryotic cells.

👉 Think about it:

• Which living things belong to prokaryotes?
• Which belong to eukaryotes?

DNA – Carrier of Hereditary Information

Inside the nucleus lies the hereditary material, the DNA molecule (deoxyribonucleic acid). DNA is built of two strands connected by cross-links and twisted into a spiral. Segments of DNA are called genes.

Genes contain instructions for making proteins, and proteins determine the traits of an organism. For example, the amount of melanin pigment produced in the skin cells, iris cells, and hair root cells defines the color of the skin, eyes, and hair.

In a gene important for the development of these traits, the information is stored about a protein that produces melanin pigment.

DNA, Chromatin, and Chromosomes

The DNA molecule inside a human cell nucleus is about 2 meters long. This raises the question: how can such a long molecule fit into a nucleus that is only about 6–10 micrometers in diameter? The answer lies in special proteins called histones.

Histones act like spools, and DNA winds around them like thread around a reel. Under a light microscope, this coiled structure looks like threads with beads on them. This form is called chromatin, where the “beads” are DNA wrapped around histones.

Chromatin can fold and organize even further into chromosomes. Chromosomes are clearly visible only during cell division.

DNA Duplication and Cell Division

Before a cell divides, each DNA molecule inside the nucleus makes a copy of itself. This means that two identical DNA molecules are formed. At the same time, the chromatin becomes more condensed and transforms into a chromosome.

A chromosome is made of two chromatids joined at one point called the centromere. Because of this, the chromosome often looks like the letter X. Each chromatid in a chromosome carries an identical DNA molecule, so they are called sister chromatids.

By duplicating DNA before division, the cell ensures that the two new cells created will each receive the same amount of DNA and carry the same hereditary information as the original (parent) cell.

Number of Chromosomes

Every species has a characteristic number of chromosomes. In human body cells, as well as in the body cells of many other organisms, the set of chromosomes is diploid, meaning that chromosomes come in pairs. This is marked as 2n.

Human somatic (body) cells contain 46 chromosomes, which means they have 23 pairs of homologous chromosomes. Chromosomes are paired according to their shape, size, and the position of the centromere.

In each homologous pair, one chromosome comes from the mother and the other from the father. Therefore, half of the chromosomes (23) are of maternal origin, and the other half (23) are paternal in origin.

Karyotype and Karyogram

The complete set of chromosomes in a cell is called a karyotype. In humans, the karyotype consists of 22 pairs of autosomes (body chromosomes) and one pair of sex chromosomes.

• In females, the sex chromosomes are XX.
• In males, the sex chromosomes are XY.

When chromosomes are arranged into pairs and ordered by size, from the largest to the smallest, the result is called a karyogram. In this arrangement, the last pair always represents the sex chromosomes.

Haploid Number of Chromosomes

Unlike body (somatic) cells, sex cells contain a haploid number of chromosomes, meaning they have only half as many as somatic cells.

The haploid number is marked with the symbol n. In humans, the haploid number is 23 chromosomes in each sex cell (egg or sperm).

Gene

A gene is a part of DNA that carries instructions important for the development of certain traits in an organism. These traits depend both on the genetic information and on the influence of the environment. Most often, genes contain information for making proteins, which play different roles in the body.

In body cells, there are two copies of each gene – one located on the chromosome inherited from the mother, and the other on the corresponding chromosome inherited from the father. These copies may be the same or slightly different. Different versions of the same gene are called alleles.

Each gene occupies a precise position on the chromosome, called a gene locus. For example, one allele may code for tall growth, and another for short growth. When the two alleles on a chromosome pair are different, the combination is called a heterozygote. When both alleles are the same, the organism is a homozygote.

How Genes Influence Cell Traits

Genes carry instructions for making proteins. Different cell types produce different proteins, which give them their specific functions. For instance, muscle cells produce proteins that allow contraction, while nerve cells produce proteins that help them transmit signals.

This means that although all the cells of an organism contain the same DNA, they express different genes depending on their role. Because of this, cells can look and behave very differently even though they share the same genetic material.

Student Task: Answer the Following Questions

1. What is the main difference between eukaryotic and prokaryotic cells in terms of genetic material?
2. Where in the cell is DNA located, and what is its role?
3. What is a gene, and what does it usually contain instructions for?
4. How does DNA fit inside the small nucleus, even though its length is about 2 meters?
5. What are chromosomes, and when are they most visible under a microscope?
6. Why does DNA duplicate before a cell divides?
7. How many chromosomes do human body cells have? How many come from the mother and how many from the father?
8. What is the difference between diploid (2n) and haploid (n) chromosome numbers? Give an example from humans.
9. What is the difference between homozygote and heterozygote? Give an example.
10. Why do different cells (like skin cells, muscle cells, and nerve cells) look and function differently even though they contain the same DNA?