Chromosomes
A chromosome is a nucleoprotein structure that generally appears like a rod-shaped structure during nuclear division. These chromosomes are tightly packed inside the nucleus of a cell and are made of DNA molecules. This DNA is responsible for carrying the hereditary information. Thus chromosomes as a whole play an important role in inheritance.
Gene is typically a unit of DNA that is located on the chromosome in a linear fashion. The occurrence of different genes on the same chromosome is called linkage. Furthermore, all genes located physically on a given chromosome are termed a ‘linkage group’.
Human Chromosomes
In humans, 23 such chromosomes occur in pairs and thus totalling 46 chromosomes. These 23 pairs can be divided into autosomes and allosomes. The first 22 pairs come under autosomes, and the 23rd pair is the allosome or sex chromosome.
Also Read: Difference between Gene and DNA
Genetic Mapping
Geneticists use ‘map unit’ to measure the distance between linked genes. It is a diagrammatic graphical representation that displays the exact arrangement, location and combination of genes in a linkage group of chromosomes. This kind of gene mapping technique plays a vital role in the Human Genome Project (HGP). The goal of this international research project is to identify and sequence all the genes that make up the human genome.
Genetic mapping is also adopted to construct linkage maps for fungi, Drosophila, maize, garden pea, etc.
23 Chromosomes and Their Functions
The major function of chromosomes is to carry the hereditary information from one cell generation to the next. DNA is the only permanent component of the chromosome and is the sole genetic material of eukaryotes. Each chromosome has several genes that code multiple proteins. Thus they determine or influence the traits of the entire eukaryote.
With advancing research techniques, it has become possible to locate and analyse the functions of all the 23 chromosomes in humans. The following table provides the details of the gene and diseases associated with each chromosome.
|
Chromosome |
Gene |
Diseases and Disorders |
|
1 |
Actin, skeletal muscle alpha chain Pancreatic Amylase Acid phosphatase Cystic fibrosis antigen Rhesus blood antigen Xeroderma pigmentosum A |
Alzheimer’s Glaucoma Galactosemia Congenital hypothyroidism Prostate cancer Breast cancer Parkinson disease |
|
2 |
Glucagon Elastin Collagen â…¢ Collagen â…£ Testis-specific É‘ tubulin Interferon â… |
Autism Gilbert’s syndrome Waardenburg syndrome Brachydactyly type D Lynch syndrome |
|
3 |
Rhodopsin Somatostatin |
Lung/breast/colon/pancreatic cancer Cataracts Von Hippel-Lindau syndrome |
|
4 |
Huntingtin protein MN blood group |
Huntington’s disease Parkinson’s disease Achondroplasia |
|
5 |
Fibroblast growth factor, acidic |
Cri du chat syndrome Cockayne syndrome Spinal muscular atrophy |
|
6 |
Insulin-dependent diabetes mellitus 1 Î’ tubulin M40 |
Diabetes mellitus type 1 Hemochromatosis Epilepsy |
|
7 |
Histone cluster A: H1, H2A, H2B Collagen type 1 alpha 2 chain Non-histone chromosomal protein 2 |
Cystic Fibrosis Williams syndrome Pendred syndrome |
|
8 |
Carbonic Anhydrase cluster DNA Polymerase β |
Burkitt’s lymphoma Werner syndrome |
|
9 |
ABO blood group |
Tangier disease Tuberous sclerosis |
|
10 |
Hexokinase â… |
Pfeiffer syndrome Barakat syndrome |
|
11 |
Insulin Catalase |
Ataxia telangiectasia Insulin-dependent diabetes mellitus type 2 |
|
12 |
Collagen â…¡, alpha 1 Salivary protein complex |
Phenylketonuria Pallister-Killian syndrome |
|
13 |
Collagen â…£ alpha 1 chain and alpha 2 chain Ribosomal RNA |
Patau syndrome Retinoblastoma Wilson’s disease |
|
14 |
T-cell leukaemia 1 Immunoglobulin heavy chain gene cluster Ribosomal RNA |
Alzheimers Burkitt’s lymphoma Multiple myeloma |
|
15 |
Ribosomal RNA |
Tay-Sachs disease Marfan syndrome Prader-Willi syndrome |
|
16 |
Nonhistone chromosomal protein 1 Haemoglobin alpha Haemoglobin zeta |
Familial Mediterranean fever Thalassemia |
|
17 |
Growth hormone |
Charcot-Marie-Tooth disease |
|
18 |
Gastric releasing peptide |
Niemann-Pick disease |
|
19 |
Green/blue eye colour Bombay phenotype |
Myotonic dystrophy Maple syrup urine disease |
|
20 |
Growth hormone-releasing factor, somatocrinin |
Alagille syndrome |
|
21 |
Ribosomal RNA |
Down syndrome Autoimmune polyendocrine syndrome |
|
22 |
Ribosomal RNA Myoglobin |
DiGeorge syndrome Chronic myeloid leukaemia |
|
X |
Haemophilia A, factor â…§ Haemophilia B, factor â…¨ Gonadal dysgenesis, XY female type |
Turner syndrome Rett syndrome Haemophilia A |
|
Y |
Pseudoautosomal segment Testicular determining factor Azoospermia-third factor H-Y antigen |
Y chromosome microdeletion XYY syndrome |
As mentioned earlier, each chromosome has several genes. The above table discusses a few genes and a few disorders associated with each chromosome.
Function of X and Y chromosomes
In humans, the X chromosome has about 800 protein-coding genes and the Y chromosome has about 70 genes. Both males and females have X chromosomes – males have one X whereas females have two X chromosomes. The X chromosome also has a role in moulding sex-specific immune responses. Whereas the Y chromosome carries a unique gene called the SRY gene that initiates the sex determination in males. It is also known as the testis-determining factor.
Also Check: Difference between Gene and Chromosome
Frequently Asked Questions
What is the difference between autosomes and allosomes?
Autosomes are chromosomes that are labelled with numbers from 1 to 22. They determine the somatic traits. Whereas allosomes are sex-determining chromosomes. They are the 23rd pair and are labelled as XY, ZW, ZO and XO. Autosomes and the female allosomes (XX) are homologous and the male allosomes are non-homologous (XY).
What are karyotype and idiogram?
A karyotype is the entire set of chromosomes of a cell or individual, as seen during mitotic metaphase. It helps to determine the size, shape, number and type of chromosomes. Idiogram is nothing but the diagrammatic representation of a karyotype. Usually, in an idiogram, the chromosomes of a haploid set of an organism are ordered in a series of decreasing sizes.
What is cytological mapping?
Cytological maps or physical maps determine the physical locations of a gene in a chromosome with the help of genetic markers. Cytological maps are more accurate in terms of genome representation than genetic maps. The genetic map provides insight into the nature of various regions of the chromosome and is comparatively less efficient.
Keep exploring BYJU’S Biology for more exciting topics
Comments