Written by Jay Ryan L. Jacutin

Chromosomes are one feature of the eukaryotic cell residing within the nucleus. Its DNA in plants and animal cells is a thread-like structure compacted in a tight manner. The DNA floats in a freeway across the cell in respect of bacteria.

They comprise DNA wherein its structural support called histones coils the chromosomes. The derivation of the term chromosomes comes from the Greek word chroma (color) and soma (body). It is due to the staining of cell structures with colorful dyes in research.

The binding of the chromosomes and histones yields nucleosomes, aiding gene regulation. Moreover, the nucleosomes coiled up to form chromatin loops, making complete chromosomes.

A chromosome contains 2 short arms, 2 long arms, and a centromere. The short arm’s denotation is ‘p’ while ‘q’ for long arms. The entirety of the structures holds or maintains themselves at the center.

The DNA encompasses the chromosomes has thousands of genes. They served as the basic physical and functional unit of heredity. They develop and maintain cells and carry the genetic information to offspring.

Humans get 2 copies of each, inherited from both parents. Most genes are the same to humanity, but only a few variations. The gene’s variation contributes to distinguishing a person’s physical features.

What are the 23 chromosome pairs?

From a diploid perspective, there are 46 in total in counting chromosomes. From a haploid viewpoint, there are only 23.

For a fact, 22 out of 23 are autosomes, and the remaining pair are sex chromosomes (X and Y). As stated, both parents contribute to one chromosome of each pair for offspring. Female obtains 2 X chromosomes while males have a single X and Y chromosomes.

The genes within are vital for the body’s development, growth, and chemical reactions. Otherwise, abnormalities persist either due to deficiency or excess. Rearrangement of its structure may also be a contributing factor:

Deletions. Missing or deleted segment; deficiency.

Duplications. Duplicated or copied component; excess.

Translocations. Either segment from 2 different or the entire chromosomes attached to another centromere.

Inversions. Discontinuity of the bonded chromosomes turned upside down, inverted. Rings. Discontinuity chromosomes formed circles with or without genetic material loss. Chromosomal abnormalities often occur in cell division:

Mitosis yields to duplication of 2 cells from the original cell. It means a cell containing 46 chromosomes divides and produces 2 identical cells. It happens throughout the body, excluding the reproductive organs.

Meiosis resulted in half number of the chromosomes only instead of the usual total of 46. Unlike mitosis, it occurs now in the reproductive organs creating egg and sperm cells.

Other contributing factors increasing the risk of abnormalities are as follows:

Maternal Age. Aging women have a higher risk of giving birth to babies with abnormal chromosomes.

Environment. No conclusive study shows the specific factors that provoke the said abnormality. There is still a possibility that they play—a role in the occurrence of erroneous processes.

What is the smallest chromosome?

Chromosome 21 is the smallest and has an approximate number of 48 million bonded bases. They represent 1.5 to 2 percent of the DNA’s entirety.

In 2000, Genome Project discovered Chromosome 21 as the second human sequenced pair. It contains 200 to 300 genes that aid in the production of proteins and have various roles in the body.

Chromosomal conditions associated with changes in the structure may yield the following:

Down Syndrome is often caused by trisomy 21, having three copies instead of 2. The extra copy disrupts the normal development of the individual. It also increases the risk of developing seizures, endocrine problems, or Alzheimer’s disease.

Core binding factor acute myeloid leukemia is due to translocation written as t(8:21). The denotation of genetic changes is somatic mutation meaning it is not inherited. Tolerating the production of abnormal leukocytes develops cancerous leukemia cells.

Other Chromosomal Conditions. Any changes in the smallest chromosomes may affect one’s health and development. It may cause intellectual disability, delayed development, and distinctive facial features. Some cases show symptoms incongruent with Down Syndrome.

What is the largest chromosome?

Meanwhile, the largest one is Chromosome 1, with an estimation of 249 million DNA bonded bases. They represent 8 percent of the entire partition of the cell’s DNA. It contains 2,000 to 2,100 genes that aid in the production of proteins and have various roles in the body.

Sudden changes in its structure may lead to chromosomal conditions:

1p36 deletion syndrome occurs when deleted genetic material happens in the ‘p’ arm. The size of the deletion varies to the affected individuals. Signs and symptoms include intellectual disability or distinctive facial features.

1q21.1 microdeletion arises from a deletion part of each cell’s ‘q’ arm. Once affected, individuals are missing with 1.35 million DNA building blocks. Individuals may have delayed development, intellectual disability, and neurological problems.

1q21.1 microduplication is a copied segment on the 2 copies of chromosome 1 in each cell. It involves approximately 1.35 million missing two bond chemical bases. Apart from developmental delays, they may encounter psychiatric disorders such as schizophrenia.

Neuroblastoma is a cancerous tumor and an associated disease due to 1p36 deletion. At present, researchers are still studying how it aids in neuroblastoma development.

Thrombocytopenia Absent Radius Syndrome caused by the deletion of 1.q21.1. It eliminates at least 200,000 DNA building blocks from the ‘q’ arm. TAR’s denotation is by the absence of radius in each forearm and platelet shortage.

Other cancers due to chromosomal changes are somatic. ‘p’ arm deletion implies a tumor in the brain and kidneys. Meanwhile, duplication in the ‘q’ arm suggests blood and bone marrow disease.

Are males XY or YY?

Male has an XY pair as they inherited the X chromosome from the mother and Y from the father.

Dissecting the difference, males have an XY pairing during XX for females. In other words, fathers can contribute X or Y while mothers always contribute an X.

In the aspect of its size, Y chromosomes are only one-third of the X size. Meanwhile, Y- carries 55 basic units of hereditary while X- contains 900.

The XY served as a sex-determination system in humans, reptiles, and plants. For a fact, the expression of Y-linked genes is only for males. The other term for the male determining gene of Y is the SRY gene.

Why do we have 2 of every chromosome?

Chromosomes are in pairs because each parent contributes one of its own. Otherwise stated, one chromosomal pair is being inherited from their biological parents. One set for the mother and the other for the father.

If there are 23 chromosomal pairs, there will be 23 from the mother and another 23 for the father. The chromosome sets may inherit the copy of each gene to their children. Hence, 22 of the said bonded chemical bases are autosomes, and the remaining is a sex chromosome.

In maintaining the exact number of chromosomes, they must undergo mitosis. The process involves the division of DNA, yielding 2 identical cells. It ensures the old and new cells have similar genetic makeup and contain the same parent cell number.

Mitosis occurs in non-reproductive somatic cells. In comparison with meiosis, it occurs in sexual reproduction.

For reproduction to come to happen, it creates 4 new daughter cells. It creates 4 new daughter cells with slight distinctive genetic make ensuring diversity. They ended as haploid, meaning they contained half the number of chromosomes.

In contrast, mitosis only splits once since they only create 2 daughter cells. No pairing of homologous chromosomes, unlike meiosis. Thus, the cells produced are identical and end as a diploid with the same number of chromosomes.

How many genes does a chromosome have?

The Human Genome Project was an international collaborative effort in studying human genes.

Based on their findings, the human genome comprises about 3 billion bonded bases. They situate themselves in the chromosomes bond chemical bases within the nucleus.

Furthermore, it specifies the estimation of genes present for about 30,000. The following are the estimation of chromosome pairs according to their number/letter:

1. About 3,000 basic units of inheritance with an estimation of 240 million base pairs. The determination of two bonded chemical bases is approx 90%.
2. An estimation of 1900 genes with 200 million base pairs. The determination of two bonded chemical bases is appro 95%.
3. More or less 1900 basic units of inheritance with 200 million base pairs. The determination of two bonded chemical bases is approx 95%.
4. Around 1600 genes with 190 million base pairs. The determination of two bonded chemical bases is approx 95%.
5. About 1700 basic units of inheritance with 180 million base pairs. The determination of two bonded chemical bases is approx 95%.
6. More or Less 1900 genes with 170 million base pairs. Determination of pairs is approx 95%.
7. An estimation of 1800 basic units of inheritance with 150 million base pairs. The determination  of  two bonded chemical bases  is  approx  95%.
8. About 1400 genes with 140 million base pairs. The determination of two bonded chemical bases is approx 95%.
9. Around 1400 basic units of inheritance with 130 million base pairs. The determination of two bonded chemical bases is approx 85%.
10. More or Less 1400 genes with 130 million base pairs. The determination of two bonded chemical bases is approx 95%.
11. About 2000 basic units of inheritance with 130 million base pairs. The determination of two bonded chemical bases is approx 95%.
12. Around 1600 genes with 130 million base pairs. The determination of two bonded chemical bases is approx 95%.
13. More or Less 800 basic units of inheritance with 110 million base pairs. The determination of two bonded chemical bases is approx 80%.
14. About 1200 basic units of inheritance with 100 million base pairs. The determination of two bonded chemical bases is approx 80%.
15. Around 1200 basic units of inheritance with 100 million base pairs. The determination of two bonded chemical bases is approx 80%.
16. About 1300 genes with 90 million pairs. The determination of two bonded chemical bases is approx 85%.
17. More or Less 1600 basic units of inheritance with 80 million base pairs. The determination of two bonded chemical bases is approx 95%.
18. About 600 genes with 70 million base pairs. The determination of two bonded chemical bases is approx 95%.
19. More or less 1700 basic units of inheritance with 60 million base pairs. The determination of two bonded chemical bases is approx 85%.
20. Around 900 genes with 60 million base pairs. The determination of two bonded chemical bases is approx 90%.
21. More or less 400 basic units of inheritance with 40 million base pairs. The determination of two bonded chemical bases is approx 70%.
22. About 800 genes with 40 million base pairs. The determination of two bonded chemical bases is approx 70%.

X. Around 1400 basic units of inheritance with 150 million base pairs. The determination of two bonded chemical bases is approx 95%.

Y. More or Less 200 genes with 50 million base pairs. The determination of two bonded chemical bases is approx 50%.

The project does not only quantify genes but to get a complete mapping and understanding. HGP optimized the data analysis to 99.99% accuracy and sequenced the entire genome. Genome refers to genes in general or the organism’s complete set of DNA.

HGP revolutionizes the diagnosis and treatment of many illnesses. It benefited both medical science and impacts on drug discovery.

Reference

Collins, F.S. & McKusick, V.A. (2001). Implications of the Human Genome Project for Medical Science. National Library of Medicine. https://pubmed.ncbi.nlm.nih.gov/11176855/

Germanna Community College (n.d.). Mitosis vs. meiosis.content/uploads/tutoring/handouts/Mitosis-vs-Meiosis.pdf

Medline Plus (2022). Chromosome 1. National Library of Medicine. https://medlineplus.gov/genetics/chromosome/1/#resources

Medline Plus (2022). Chromosome 21. National Library of Medicine. https://medlineplus.gov/genetics/chromosome/21/#conditions

Medline Plus (2022). How many chromosomes do people have?     National Library of Medicine. https://medlineplus.gov/genetics/understanding/basics/howmanychromosomes/\

Medline   Plus    (2022).    What   is    a.    chromosome?       National   Library   of   Medicine. https://medlineplus.gov/genetics/understanding/basics/chromosome/

National Human Genome Research Institute (2022). Chromosome abnormalities fact sheet. https://www.genome.gov/about-genomics/fact-sheets/Chromosome-Abnormalities-Fact- Sheet

National     Center     for     Biotechnology     Information     (1998).      Chromosome     map.

https://www.ncbi.nlm.nih.gov/books/NBK22266/

National   Center   for   Biotechnology   Information   (2009).    Appendix    F    chromosomal abnormalities.

National Human   Genome   Research   Institute   (2022).   Human   genome   project   FAQ.

https://www.genome.gov/human-genome-project/Completion-FAQ

National    Human    Genome   Research   Institute    (2022).    Y    chromosome   infographic.

https://www.genome.gov/about-genomics/fact-sheets/Y-Chromosome-facts

Szalay, J. & Dobrijevic, D. (2022). Chromosomes: Facts about our genetic storerooms. Live Science.

Your Genome (2022). What is chromosome?