Main Difference – Chromosome vs Chromatid
DNA carries the genetic information of an individual through its progeny. DNA exists in a double-stranded structure inside the cell nucleus. Both these DNA strands are coiled together to form a double-helix. Chromosomes are the thread-like structures which consist of a tightly coiled double stranded DNA molecule around histone proteins; chromatid refers to either of the two thread-like strands which a chromosome divides longitudinally during the cell division. The key difference between chromosome and chromatid is their structure; a chromosome has the most condensed structure of DNA, whereas a chromatid has the unraveled condensed structure of DNA.
This article studies,
1. What is a Chromosome
– Definition, Characteristics, Classifications
2. What is a Chromatid
– Definition, Characteristics
3. What is the difference between Chromosome and Chromatid
What is a Chromosome
A chromosome is a structure where DNA is packed into its highly condensed form. It consists of long DNA chains which are associated with proteins. Most prokaryotes contain a freely floating, single, circular chromosome. It is located at nucleoid. The prokaryotic chromosomes do not contain introns. Their genes are expressed as groups called operons. Prokaryotes consist of histone-like proteins associated with their DNA. Other than that, bacteria contain extra-chromosomal elements called plasmids. In eukaryotes, chromosomes are located in the nucleus which is enclosed by a separate membrane. DNA is tightly wound around histone proteins. Strands of DNA are about 150-200 long and wrap twice around a core consisting of eight histone proteins. The structure is called a nucleosome. This winding provides structural support and allows the control of gene activity.
In humans, 46 individual chromosomes are found: 22 pairs of autosomes and two sex chromosomes. A functional chromosome has an origin of replication, centromere, and telomeres. Four chromosome types can be identified based on the centromere position. They are Telocentric, Acrocentric, Submetacentric and Metacentric chromosomes. Nuclear division can be arrested at metaphase in order to study chromosomes. The process, where abnormalities of the chromosomes are identified, is known as karyotyping.
Figure 1: Spectral karyotype
What is a Chromatid
A chromosome is composed of one DNA molecule. During the S phase of the cell, DNA gets doubled in its amount in order to enter the cell division. A new copy of a DNA strand is formed based on the genetic information carried by the existing strand. However, the number of chromosomes within the cell remains the same. Thus, each chromosome contains two copies of DNA strands. One of the DNA strands in a chromosome is referred to as a chromatid. Hence, a chromatid is a single DNA strand. It has a thread-like structure and consist of chromatin fibers. DNA wraps around proteins called histones and coil further to form chromatid fibers.
The two chromatids found in a chromosome can be identified as a sister chromatid pair. Sister chromatid pairs are joined together by the centromere. Sister chromatids are separated during anaphase. Anaphase is the third stage of the M-phase of the cell cycle. Chromatids are found in their most condensed state at the anaphase. Separated sister chromatids are then known as daughter chromosomes. Sister chromatids are identical in the information they carry. Therefore, sister chromatids are considered as homozygous. However, mutations can occur during the replication. A mutation in the newly formed strand thus makes sister chromatids heterozygous. Maternal and paternal homologous chromosomes pair during sexual reproduction. This kind of a chromatid pair is referred to as non-sister chromatids.
Figure 2: Macrostructure of DNA
Difference Between Chromosome and Chromatid
Condensation
Chromosome: DNA is condensed 10, 000 times to form a chromosome. Thus, a chromosome is the most condensed form of DNA
Chromatid: DNA is condensed 50 times to form a chromatid. Thus, a chromatid is less condensed than a chromosome.
Content
Chromosome: A chromosome consists of a single, double-stranded DNA molecule.
Chromatid: A chromatid consists of two DNA strands joining together by their centromere.
Structure
Chromosome: A chromosome is a thin, ribbon-like structure.
Chromatid: A chromatid is a thin and long, fibrous structure.
Genetic Material
Chromosome: Homologous chromosomes are not identical. They might have different alleles of the same gene.
Chromatid: Homologous sister chromatids are identical.
Stage
Chromosome: Chromosomes appear in M phase.
Chromatid: Chromatids appear in the interphase.
Function:
Chromosome: Chromosomes are involved in the distribution of genetic material.
Chromatid: Chromatids are involved in metabolism and other activities of the cell.
Conclusion
A chromosome consists of a single DNA molecule whereas a chromatid consists of two identical DNA strands joined together by the centromere. Chromosomes generally participate in the distribution of genetic material at the nuclear division. Chromatids participate in metabolism and regulation of gene expression. Nevertheless, DNA is condensed 10,000 times in a chromosome while it is condensed 50 times itself in a chromatid. Thus, the key difference between a chromosome and a chromatid is in the level of condensation.
Reference:
1. Higgins, N. P., Chromosome Structure. ENCYCLOPEDIA OF LIFE SCIENCES. 2015 http://smcg.ccg.unam.mx/enp-unam/03-EstructuraDelGenoma/cromosomeStructure.pdf 08 Feb. 2017
2. “Chromosome”. National Human Genome Research institute. 08 Feb. 2017 https://www.genome.gov/26524120/chromosomes-fact-sheet/
Image Courtesy:
1. “Sky spectral karyotype” from National Human Genome Research Institute – Found on:National Human Genome Research (USA) – copied from wikipedia:en. (Public Domain) via Commons Wikimedia
2. “0321 DNA Macrostructure” By OpenStax – (CC BY 4.0) via Commons Wikimedia
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