A chromosome is a thread-like structure present in the nucleus or nuclear region of the cytoplasm that is made up of a single molecule of DNA (Deoxyribonucleic acid) and proteins, carrying some or all genetic materials of an organism.
- These chromosomes are only visible under the light microscope during the metaphase of the cell cycle where the chromosomes line up in the center of the cell.
- Prokaryotes have a single circular chromosome that is organized in the nucleoid region of the cytoplasm as they do not have a localized nucleus.
- Extrachromosomal DNA might be present in some prokaryotes in the cytoplasm in the form of plasmids. These are involved in horizontal gene transfer.
- In the case of eukaryotic organisms, the chromosome is associated with packaging proteins that prevent the chromosomes from becoming unmanageable.
- Eukaryotic chromosomes are packaged into a condensed structure called chromatin by the chromatin fibers. Because the chromosomes are condensed, it is possible to fit the very long chromosomes into a nucleus.
- An additional chromosome might be present outside the nucleus in the mitochondria and chloroplasts of some eukaryotes.
- The structure of chromosome changes during different phases of the cell cycle. Chromosomes might uncoil, replicate, and divide during different phases of cell division.
- Chromosomes in humans are divided into two types; autosomes which are also called body chromosomes and allosomes, which are also called, sex chromosomes.
- Humans have 23 pairs of chromosomes with 22 pairs of autosomes and a pair of sex chromosomes with a total of 46 chromosomes in total.
- These chromosomes carry all the genetic materials necessary for various purposes ranging from the formation of proteins to division and reproduction.
- Chromosomes are highly regulated structures that are responsible for genetic diversity among sexually reproducing organisms. In asexual reproduction, the chromosomes are duplicated, and equal genetic material is transferred to the daughter cells.
- During sexual reproduction, genetic variability is brought about by the process of crossing over during meiosis.
- Deformities in chromosomes might lead to various chromosomal disorders, and sometimes mutations in the genes might even lead to cancer.
A chromatid is an identical half of a duplicated chromosome. After duplication of a chromosome, two identical halves are formed, each of which is called a chromatid.
- Chromatids are less condensed than chromosomes as these are formed once the chromosome has uncoiled.
- The chromatids that are formed from the same chromosome and are connected in the center by a centromere and, are called sister chromatids.
- However, chromatids might also occur in the form of non-sister chromatids where the two chromatids from two different chromosomes are connected via chiasmata for the exchange of genetic material.
- Chromatids exist only during cell division, where the chromosome replicates to increase the mass of the DNA. The DNA is then separated into two chromatids, which of which contain the same genetic information.
- As the cell division progresses, the chromatids separate and develop to form individual chromosomes.
- Thus, chromatids are temporary structures that only exist to aid the process of chromosome duplication and separation.
- Because chromatids are identical, they are also called to be homozygous. In some cases, however, due to mutations some changes might occur in one or both of the chromatids, resulting in heterozygous chromatids.
- Chromatids are particularly important structures during the meiosis of cell division. During the prophase I of meiosis I, the non-sister chromatids (one from the paternal chromosome and other from the maternal chromosome) form charismata to exchange the genetic material.
- This event is crucial in sexual reproduction as it ensures the genetic variability and thus diversity within the species.
- Chromatids, unlike, chromosomes are not capable of transcribing proteins.
Key Differences (Chromosome vs Chromatid)
|Basis for Comparison||Chromosome||Chromatid|
|Definition||A chromosome is a thread-like structure present in the nucleus or nuclear region of the cytoplasm that is made up of a single molecule of DNA and proteins, carrying some or all genetic materials of an organism.||A chromatid is an identical half of a duplicated chromosome. After duplication of a chromosome, two identical halves are formed, each of which is called chromatids.|
|Compactness||Chromosomes are more condensed than chromatids.||Chromatids are less condensed than chromosomes.|
|Consists of||A chromosome consists of a single, double-stranded DNA molecule.||Chromatids are two molecules of double-stranded DNA joined together in the center by a centromere.|
|Structure||Chromosomes have a thin ribbon-like structure.||Chromatids have a thin and long fibrous structure.|
|Nature||Homologous chromosomes are not identical to each other.||Homologous or Homozygous sister chromosomes are identical.|
|DNA||DNA in a chromosome is tightly packed.||The DNA molecule is unwounded and thus is free.|
|Present||Chromosomes are present in all cells throughout their life.||Chromatids are formed during the interphase and exist until the metaphase of cell division.|
|Centromeres||Chromosomes do not have centromeres.||Chromatids are connected to the center by a centromere.|
|Duplication||Chromosomes are capable of replication or duplication.||Chromatids cannot replicate or duplicate.|
|Protein synthesis||Genetic information in chromosomes can be transcribed to produce protein molecules.||Chromatids are not involved in macromolecule synthesis like protein formation.|
|Function||Chromosomes carry genetic material and thus are involved in the transfer of genetic material through different generations of organisms.||Chromatids help to maintain the proper amount or number of DNA in the cell after cell division.|
References and Sources
- National Human Genome Research Institute. 13 July 2020. https://www.genome.gov/genetics-glossary/Chromosome
- National Human Genome Research Institute. 13 July 2020. https://www.genome.gov/genetics-glossary/Chromatid
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