Genes and chromosomes

gene is a portion of deoxyribonucleic acid (DNA) that codes for a protein or RNA. chromosome is a nuclear structure that corresponds to the packaging of DNA where a number of genes are found. In other words, the chromosome is a large portion of DNA, the molecule in charge of the genetic information of living beings, which is expressed through genes.

DefinitionFunctional unit of inheritanceLinear DNA packaging structure
LocationOn the chromosomesIn the cell nucleus
CompositionADNChromatin: DNA and proteins
TypesStructural genes

Regulatory genes

Specialized genes

Constitutive genes


According to function in the cell:

  • Somatic or autosomal chromosomes
  • X and Y sex chromosomes

According to the location of the centromere:

  • Metacentric chromosomes
  • Submetacentric chromosomes
  • Acrocentric chromosomes
  • Telocentric chromosomes
ExamplesACTB gene: actin gene.

BRCA1 gene: tumor suppressor gene.

INS gene: insulin gene.

Homo sapiens : 46 chromosomes

Mus musculus : 40 chromosomes

Gorilla gorilla : 48 chromosomes

What are genes?

Genes are the functional units of heredity. A gene is defined as a segment of the DNA sequence corresponding to a protein, a set of protein variants, or structural RNA molecules that do not produce proteins.

The Human Genome Project determined that humans have 20,000-25,000 genes in their genome in the 3 billion base pairs of DNA. The fruit fly Drosophila melanogaster has approximately 14 thousand genes in 137 million base pairs and the Arabidopsis thaliana plant has approx. 26 thousand genes in 142 million base pairs of DNA.

Genes in eukaryotic cells present:

  • Promoter region : DNA sequence that precedes the sequence that codes for a protein. It is the region that controls the transcription of the gene.
  • Coding region : DNA sequence transcribed into mRNA to later be translated into an amino acid sequence.
  • Exons: DNA sequences that occur in the definitive messenger RNA.
  • Introns : intermediate sequences that are removed in messenger RNA.

Gene types

Genes can be of the following types:

  • Structural genes : such as the ACTB gene that encodes the actin protein, important in the cytoskeleton of the cell.
  • Regulatory genes : they code for proteins that regulate the transcription of other genes.
  • Specialized genes: genes that are only expressed in certain cells; for example the globin gene in red blood cell precursors.
  • Genes maintenance or constitutive (of the English housekeeping genes ) are genes whose transcription is constant in the cell and that meet the basic functions thereof.
  • Pseudogenes : they are non-functional genes, the result of the accumulation of mutations.

Genes function

Genes are the elements that contain the information that determines the characteristics of the species. They also control the development and functions of cells. For example, the BRCA1 gene on human chromosome 17 encodes a protein that maintains genome stability and acts as a tumor suppressor.

What are chromosomes

Chromosomes are threadlike structures found inside the nucleus, made up of DNA and proteins. The eukaryotic cells of each species of a living being possess a characteristic fixed and constant number of chromosomes. For example, Homo sapiens has 23 pairs of chromosomes or 46 chromosomes.

Chromosomes are visible under a microscope when cells begin to divide. In a given species, chromosomes can be identified by their number, size, centromere position, and banding pattern. The karyotype is the number and appearance of human chromosomes.

In organisms where there is sexual reproduction, the sex cells or gametes have only one set of the chromosomes of the species, that is, they are haploid. For example, in humans, eggs have 23 chromosomes and sperm also have 23 chromosomes.

The number of chromosomes is not related to the complexity of the organism, for example, in somatic cells of humans there are 46 chromosomes, in mice 42, in bovines 60.

Chromosome structure

Each chromosome has:

  • a narrowing called the centromere;
  • end ends known as telomeres;
  • two arms, a small one or p-arm (from the French petit ) and a long one or q-arm .

Chromosome types

In living beings where two sexes differ, chromosomes are classified into:

  • Somatic chromosomes : are those identical chromosomes between the individuals of a species, regardless of their sex. In humans there are 22 pairs of somatic chromosomes also called homologous chromosomes .
  • Sex chromosomes : they are a pair of chromosomes that determine the sex of the individual. In humans, a pair of X chromosomes (XX) determines that the individual is female, while an X chromosome and a Y chromosome (XY) determine that the individual is male.

Additionally, chromosomes can be classified according to the position of the centromere in:

  • metacentric chromosomes : the centromere is in the middle of the chromosome;
  • submetacentric chromosomes : the centromere is located at one end;
  • acrocentric chromosomes : the centromere is very close to the extreme end, resulting in a very small p-arm;
  • telocentric chromosomes : the centromere is exactly at the extreme.

Chromosome function

The main function of chromosomes is the packaging of DNA within the nucleus . Human DNA comprises more than 3 billion base pairs, which if it could be extended would be 2 meters long, but the nucleus is barely 0.000006 meters long!

Chromosomal abnormalities

Chromosomal disorders where there is a change in the number of chromosomes of the species is known as aneuploidy, among which we have:

  • Trisomy : given by the presence of an extra chromosome in the cell. For example, Down syndrome or trisomy 21 is characterized by having three copies of chromosome 21, instead of the two normal copies.
  • Monosomy : given by the absence of a chromosome in the cell. For example, Turner syndrome, where there are only 45 chromosomes with a single X chromosome.

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