The simple answer to how many chromosomes do humans have is that human beings have 23 pairs of chromosomes. It makes a total of 46 chromosomes.
Out of these 23 pairs, 22 pairs are consisting of autosomal chromosomes and the 23rd pair of chromosomes represents the sex chromosomes.
Chromosomes are the basic part of the inheritance and are responsible for the transfer of traits from parents to offspring. X and Y chromosomes are responsible for the determination of the sex of the next generation.
What are chromosomes?
Chromosomes are hereditary units and are composed of genes. Chromosomes are thread-like structures that are present in the nuclei of humans, animals, and plant cells.
Word “Chromosome” is originated from the Greek language. Chrome means color and soma means body. So, the chromosome is collectively meaning “colored body”.
This term was attributed to this hereditary structure because of specific staining of chromosomes when dyes are applied and the fact was observed by some scientists.
While talking about the structural makeup of chromosomes, they are made up of protein and Deoxyribonucleic acid (DNA) molecule.
Unique characteristics that are attributed to each individual are carried as a message hidden in DNA that is transferred from parents to the children.
23 pairs of chromosomes that make a total of 46 chromosomes are the actual source to carry the genetic information from parents to child through the process of heredity.
Details that are present in the genetic message carried by DNA - that forms chromosomes - are responsible for the unique traits of each individual that are completely different from others.
How many chromosomes are present in the cell of an organism – whether human beings or plants, determine in which sense and to which extent they will be different from each other.
For example, if we consider some species of insects, they may have just one or two chromosomes while some larger animals such as giraffes have 31 pairs of chromosomes making a total of 62.
Similarly, some other animals yet have a varied range of chromosomes. All species, including humans, have a diploid number of chromosomes.
It means that chromosomes are present in form of matching pairs, you can compare them with a pair of socks or gloves that are exactly similar to each other and serve hand in hand to perform their function.
In each pair, one chromosome is inherited from each parent during the process of fertilization. While considering the number of chromosomes in the cells of human beings, each human cell has 23 pairs and a total of 46 chromosomes.
Autosomal chromosomes: While elaboration further, among these 23 pairs, 22 pairs are referred to as autosomes as they have nothing to do with the determination of sex of the offspring.
Sex chromosomes: One pair of chromosomes that is consisting of X-chromosome and Y-chromosome are known as sex chromosomes and this only pair defines the sex of offspring.
When there is any problem in this pair of chromosomes, it results in sex-linked problems that are transferred from generation to generation, such as hemophilia is such a medical anomaly that expresses itself in generations if present in the parents.
In this XY pair, the X chromosome is donated by a female parent while the Y chromosome comes from the male parent. DNA and proteins are the structural units of chromosomes while a special section of DNA is referred to as a Gene.
Genes are present as gene pairs at the special points that are known as loci. As a whole, all the genetic material is named as genome and genome is the designing material of offspring.
Nowadays, several chromosome-linked diseases are being treated because of deep genetic study. When genetic studies are done for various individuals, it allows the scientists that are studying genetics to find out the genes that are responsible for different medical conditions.
Studying the genetic makeup of human beings, helps scientists to come up with specific treatments for lethal diseases that are specifically targeting that person’s genome. Mostly the disorders that are being treated or are aimed to be treated are genetic disorders such as sickle cell anemia or thalassemia.
Other than these two disorders, there are several disorders that are caused by the problems in genetic sequence, precisely known as “Genetic error”. One such example is diabetes in which the genes that are responsible for insulin synthesis are not properly sequenced and hence it results in Diabetes mellitus.
A better treatment strategy is only designed if you know the disorder and its roots deeply and clearly. That is the reason, scientists and researchers are focusing the genetic study more than before to come out with targeted treatments.
Discovery of chromosomes
How many chromosomes are present in the human body and how are they arranged are the points to be considered for understanding the problems and designing the treatments.
Chromosomes are too small to be seen by the naked eye and hence the scientists who study heredity, look at cells and cell materials in the microscope. The first cell was looked at by microscope in the late 1800s and at that time, cell study was quite a striking discovery.
Initially, there were no details available regarding the structure and function of a cell and the cell particles, however, with the advancement in cellular studies, now almost every detail regarding cells is in knowledge.
Methodology and technology kept on developing and in the 20th century, scientists had a much better understanding of nuclear material consisting of chromosomes.
The chromosomal study gave scientists a much better and clear vision about the relationship between the problems during inheritance and their expression in the generations.
Structure of chromosomes
For a better understanding of chromosomes and genes and their inheritance in generations, the first step is to understand the structure of chromosomes.
If we consider an individual chromosome, it’s too small to be seen with the naked eye. By using a microscope initially and an electron microscope lately, scientists came out with a clear structural study of this inheritance material.
Two major components that form a chromosome are protein and deoxyribonucleic acid – DNA. One DNA molecule is used in the formation of a single chromosome and DNA is present in coiled form around the larger protein – histone protein.
What if DNA is not coiled around protein?
Well, DNA is composed of smaller subunits and one DNA can have a length of 6 feet if it is placed uncoiled. For better fitting in the cell nucleus, DNA is coiled around the protein and hence can be fitted in the nucleus.
While talking about the structure of DNA, it is present as a double helix or like two springs that are coiled around each other. Either side of this twisted helix is consisting of a nucleotide base and a backbone made up of sugar-phosphate.
Four main nucleotide bases that are taking part in the structure of DNA are:
Adenine – abbreviated as A
Guanine – abbreviated as G
Cytosine – abbreviated as C
Thymine – abbreviated as T
The structural assembly of these bases in the helix plays the main role in the formation of a double helix – a characteristic shape of DNA. Bases are assembled in such a way that adenine is bonded with thymine and cytosine is present in front of guanine forming a link in between them.
If there is any problem in the linking of nucleotide bases, such as if A is not forming a linkage with T, or G is not linked with C, there will be a mess during the process of mitosis. Mitosis is the actual process that forms the base of cell division.
During mitosis, DNA that is present in the form of a double helix is uncoiled, and the two uncoiled strands then form new strands complementary to each of them.
Codons: Genetic Codons are the sequence of base pairs that are the basic recipe for the synthesis of proteins. Three base pairs are combined to form a single codon and different amino acids – that are building blocks of proteins – are then formed by the instructions present in form of codons.
For example, amino acid glycine is formed by a codon that has a nucleotide base sequence as GGG, i.e., three guanine bases are making a codon.
The recipe for each protein is present in a gene and the human genome is the basic responsible factor for the synthesis of proteins in the body.
Proteins are then assigned different functions in the body such as keratin protein is taking part in the manufacturing of hair and insulin is being used for controlling blood sugar level.
Almost all the functions that are being carried out in the body are involving the use of proteins. For instance, enzymes, which are the major catalysts in the biochemical reactions taking place in the body are proteins by nature.
Hemoglobin is a major component of blood and antibodies that make the defense system of the body are also proteins. Thus, it can be said that chromosomes – basic hereditary material – are composed of two main constituents.
1. Proteins – known as histones
2. Deoxyribonucleic acid (DNA)
Centromeres and telomeres
While chromosomes are the long DNA molecules that are coiled around the histone protein for the integration of the structure, the chromosome itself has some other parts.
Centromere: If we look at the shape of the chromosome, it has a constricted region in the center or away from the center, and that constricted part is known as the centromere.
No matter it is named as centromere, it’s not present in the center of the chromosome as it can be misunderstood by the name, it can sometimes present at the peripheral region of a chromosome.
Parts of chromosomes that are present on both sides of the centromere are referred to as arms and hence each chromosome has two arms.
Role of centromere: The role of the centromere in the chromosomes is to keep the chromosome in an aligned form as it goes through the complicated steps involved in the cell division.
During mitosis, when the chromosomal copies are being prepared, the centromere is the region that aids in the attachment of each half of replicated chromosomes, which are named as sister chromatids.
Telomeres: At the tips of each arm of the chromosome, the repetitive DNA stretches that are protecting the tips are known as telomeres.
The integrity of chromosomes is necessary for the proper functioning and division of the cell and if this integrity is not maintained, the cell has to suffer a lot.
In several cells, telomeres sometimes start losing their DNA during each cell division, and unfortunately if all the DNA is lost like this, cell replication isn’t possible anymore and it dies.
Especially those cells who are continuously in a phase of division, such as WBCs, have some special type of enzymes that protect them from losing DNA of telomeres during cell division. In absence of these protective enzymes, it would not have been possible for cells to keep dividing.
Although in several anomalies, telomeres are lost but in the case of malignant tumors, they are usually protected and help in fueling the unchecked growth of cells making the disease life-threatening.
Function of chromosomes
The growth and reproduction of an organism aren’t possible if the cells are not constantly dividing to generate the newer cells. When new cells are formed, they replace the older and dying cells of the body.
Yet for the transfer of genetic material from one cell to another, its necessary for genetic material to remain intact. As DNA is present in chromosomes, thus chromosomes are the defining parts of the nucleus that genetic material is intact and being distributed in other cells during cell division.
Although there are checkpoints to keep all the processes in control, yet it happens sometimes that errors in the genetic material or chromosomal division occur resulting in genome-linked disorders.
What happens when an error occurs in chromosomal division or distribution?
If some changes occur by chance in the count and the structure of the chromosome, it results in serious disorders such as leukemia is the perfect example of the chromosome-linked disorder.
If an exact number of chromosomes are not transferred from parents to children, for instance, one or two chromosomal pair is missing, the property that will be defined in the children by those chromosomes isn’t developed.
An example of such kind of chromosomal problem is Down’s syndrome in which offspring receive an additional copy of chromosome number 21. In normal human beings, there are two copies of chromosome 21 are present.
Inheritance of chromosomes
Traits are transferred from parents to children via chromosomes. All the genetic material is confined in the genes and during the process of reproduction, each parent donates a copy of chromosomes so that offspring is having the traits from both of them.
Then there is a function of dominant and recessive genes that defines further that trait of which parent will be exhibited and which parent will be suppressed in the children.
Mitosis: Mitosis is the process that controls the division of somatic cells while producing the diploid cells (2n).
Meiosis: Meiosis is also the process of cell division but is present only in gamete cells producing the four haploid gametes.
Here is interesting information that only the female gamete cells keep their mitochondria intact while sperm cells of males don’t do this. Thus, the mitochondrial DNA of offspring is only donated by the female parent and not the male.
How many genes are present in the human genome?
The human genome is a full set of DNA and the DNA is the hereditary unit that contains all the genetic instructions required for the development of properties of an individual.
DNA is a double-stranded helix in which each strand is composed of four types of nucleotide bases. These bases are Adenine, guanine, cytosine, and thymine.
Adenine is present opposite to the thymine and is linked with it while guanine and cytosine are paired together. DNA double helix remains its structure intact with the help of a phosphate bridge that acts as a backbone.
When collectively considered, the human genome contains about3 a billion of these nucleotide base pairs. All these base pairs are confined in just 23 pairs of chromosomes and that is the miracle of nature.
A single chromosome is containing thousands of genes, and all the instructions for the synthesis of proteins are present in these genes. approximately 30,000 genes present in the human genome make about 3 proteins.
Number of chromosomes in prokaryotes and eukaryotes
How many chromosomes are present in human or other cells either prokaryotes or eukaryotes? Human beings have diploid chromosomes that mean each cell is having 23 pairs of chromosomes making a total of 46.
While human beings have diploid chromosomes, some species of plants may have hexaploidy chromosomes it means they have 6 copies of 7 different chromosomes and it makes a total of 42 chromosomes.
Generally, all the members belonging to a particular eukaryotic species possess the same number of chromosomes in their nuclei. While chromosomes that are present in mitochondria or plasmid-like smaller chromosomes can be varied in number. This kind of chromosome may be present in multiple copies in each cell.
All those species that make their offsprings by sexual reproduction have somatic body cells that are diploid [2n], i.e., they have chromosomes in pairs. For example, human beings have 23 pairs that make a total of 46.
When reproduction occurs, children get one pair from each parent thus making their total of 46. While gamete cells are haploid because two gametes are combined during the process of sexual reproduction and these haploid chromosomes are produced during the process of meiosis of a normal diploid germ line cell.
When meiosis occurs, smaller parts between the chromosomes of female and male parents are exchanged to create the new chromosome.
Thus, children are getting the traits from both parents, and the property of which parent is dominant is represented in the children while masking the other one.
Haploid chromosomes from each gamete cell when combined in the form of a zygote, the diploid organism is created. Even some species of both plants and animals that are polyploid means such species are having more than two sets of chromosomes.
Mostly tobacco and wheat plants, which are important agricultural plants are polyploid although their ancestors were not like this.
For example, pasta and bread wheat are having tetraploid and hexaploidy chromosomes while opposite to them, wild wheat has diploid chromosomes. Thus, variations are present in the different species.
How many chromosomes are in a human gamete?
Well, that is an interesting fact that although all the other cells present in the human body are having 23 “pairs” of chromosomes, human gamete cells are exceptional in this regard.
Gamete cells have just 23 chromosomes that make them haploid cells. The total number of chromosomes that are present in a single set is represented by n that is a symbol of haploid.
While 2n shows the two sets of chromosomes and so on. For instance, somatic cells have 2n while gamete cells in human beings are having the n number of chromosomes.
The reason for this exception is, when male and female gamete is combined during the process of fertilization, they form a zygote with 46 chromosomes. If there are already 46 chromosomes in gamete cells, the zygote will be having a total of double these numbers.
Haploid gametes are the result of meiosis, a division process that generates the cells each with half number of chromosomes as the parent cell. However, some species may have a part of their life cycle as haploid and the other half as diploid, such as algae.
While some other organisms, for example, ants may live as haploid species throughout their life.
How many chromosomes do Down syndrome have?
There are some inheritance or chromosome-related disorders that occur because of some irregularities in the number of chromosomes or structural issues. A similar type of disorder is Down’s syndrome.
Normally, when a baby is born, he is having 46 chromosomes in each cell. But a baby with Down’s syndrome has an additional copy of chromosome 21 that is an abnormality. This expression of an extra copy of the chromosome is known as trisomy in medical terms.
As trisomy is occurring in chromosome 21, it is also narrated as Trisomy 21, i.e., an extra copy of chromosome number 21.
This additional and undesired copy of chromosome 21 results in the abnormal development of both the brain and the body of the baby. This abnormal development causes both kinds of problems including mental and physical.
Although there are similarities in the appearances of all the people having Down’s syndrome, yet there are differences in the abilities that they have.
The main difference between people with Down’s syndrome from the normal human beings is that they have low IQ levels and speech problems.
Role of chromosomes in Down’s syndrome
Down’s syndrome has three variants according to the problem in chromosome 21 although all these types are too similar to be differentiated by symptoms and characteristics. These types can be defined as:
1. Trisomy 21
This is the most common type of Down’s syndrome and about 95% of people with Down syndrome are having this type. In trisomy 21, each cell of the body is having an additional copy of chromosome 21 hence 3 copies of this chromosome are present in all the cells that are represented by the name.
2. Translocation Down’s syndrome
A smaller number of people are having translocation Down’s syndrome and it is present in almost 3% of people. When there is an extra part or a full extra chromosome 21 is present in the cells but it’s not present as a separate 21 chromosome.
It is translocated to a different chromosome and hence named as translocation Down’s syndrome.
3. Mosaic Down syndrome:
Although present rarely – in up to 2% of people – mosaic Down syndrome is characterized by having regular two copies of chromosome 21 in some cells and an extra coping in others. Thus, it’s a mixture of normal and abnormal chromosome 21 and hence named mosaic Down’s syndrome.
Those people who are suffering from mosaic Down syndrome have some characteristics or features in common with other types of Down’s syndrome while differentiating in some because of cells with normal copies of chromosome 21.
Frequently asked questions
Several questions are relevant to how many chromosomes in humans. Some of them have been answered in this section briefly yet precisely.
1. What are 24 chromosomes?
Although it sounds weird that human beings have 24 chromosomes. But it doesn’t mean that human beings are having 24 pairs of chromosomes making 48 chromosomes in total.
It’s about the type of chromosomes that human beings have. Human beings – known as Homo sapiens – have 24 chromosomes in the sense that 22 chromosomes are autosomal, one is -chromosome and the 24th is Y-chromosome. Both X and Y chromosomes are sex chromosomes.
2. How many sexes do humans have?
Human beings normally have 2 sexes that are male and female. Sometimes, due to chromosomal errors, there may be a third type of sex in human beings that is called transgender.
However, based on reproductive cells and sexual reproduction, females are characterized by ovules while males are characterized by having spermatocytes.
3. Do humans have 48 chromosomes?
Normally, human beings have 46 chromosomes that are present in form of 23 chromosomal pairs. Out of these 23 pairs, 22 pairs of chromosomes are autosomal while the last pair is consisting of sex chromosomes.
However, if they’re a human being is having 48 chromosomes, it means it’s not a normal condition. Chromosomal aberration may result in such manipulations that express themselves in form of some chromosome-linked disorder.
4. What gender is the XXY chromosome?
While observing the chromosomal makeup of a normal human being, it’s obvious that a female offspring has two X chromosomes and a male offspring has one X and one Y chromosome.
In some conditions, the child is having XXY chromosomes which means he is carrying an extra chromosome. This disease is known as Klinefelter syndrome and although the child is having male gender yet female characteristics such as enlarged breasts are apparent in such a child.
5. Can females have XXY syndrome?
The presence of a Y-chromosome in the set shows that the child will be male. Normal males have XY chromosomes while the X chromosome is donated by the female parent and the Y chromosome comes from the male parent.
But, if a child has XXY chromosomes, it means although he is male, yet he will be having some characteristics that are attributed to females. However, a female can’t have XXY syndrome.
The simplest answer to the question that how many chromosomes do humans have is that human beings have 23 pairs of chromosomes.
Having 23 pairs of chromosomes makes a total of 46 chromosomes and out of these 23 pairs, 22 pairs are consisting of autosomal chromosomes while the last or 23rd pair represents the sex chromosomes.
Chromosomes are the basic part of the inheritance and are responsible for the transfer of traits from parents to offspring. X and Y chromosomes are responsible for the determination of the sex of the next generation.
Chromosomes are made up of deoxyribonucleic acid (DNA) and DNA threads are coiled around large histone proteins. DNA is consisting of genes and all the details about heredity characters are present in the genetic message.
Mitosis and meiosis are the processes that are involved in the cell division and development of new organisms. Mitosis produces cells with diploid chromosomes that are copies of parent cells.
Meiosis is the process of cell division that is present in genetic cells and it results in four haploid copies of chromosomes. Thus, haploids from both parents when combined result in a new diploid set of chromosomes.
If there is any irregularity in the number or structure of chromosomes, it is expressed in form of some chromosomal-linked syndromes such as Klinefelter syndrome in which XXY chromosomes present in the male represent themselves in form of some female characteristics.