Chromosomes do contain proteins and these proteins are responsible for their compact and condensed structure.
Chromosomal proteins are necessary to organise the extensive linear DNA present in both the prokaryotic and eukaryotic cells. The condensation takes place in several steps and different kinds of proteins are responsible for different level of compaction.
In eukaryotes, the most common type of proteins involved in chromosomal condensation are histones and some non- histone proteins, such as condensins. Whereas, prokaryotes like bacteria and archaea have different sets of protein, of which some are analogous to that of the eukaryotic proteins while others can be unique to a specific genus.
Are chromosomes made of proteins?
Chromosomes are highly condensed form of DNAs, that are super coiled with the help of some DNA binding proteins. So, chromosomes are composed of both nucleotides and proteins.
DNA binding proteins can be broadly classified into two categories, the histone proteins and the non- histone proteins. Proteins of both the categories take part in DNA condensation to form chromatin networks that undergo further condensation during cell division to form chromosomes.
Histones are DNA binding proteins that help in chromosomal packaging. A single cell contains a large amount of histone proteins. The histone core is composed of eight histone proteins, two molecules each of H2A, H2B, H3, and H4. Almost 146 nucleotides of DNA get wrapped around the histone core to form a nucleosome. The nucleosomes are present at a regular interval of base pairs.
The condensin proteins, similar to the cohesion proteins, are a complex of five proteins, highly conserved in eukaryotes. They play a central role in the assembly of DNA into chromosomes during the mitotic phase of the cell cycle. Two types of condensins are present in eukaryotes that perform different roles during the cell division in both somatic and gametic cells. Both condensin I and condensin II possess subunits that belong to the structural maintenance of chromosomes or the SMC family, along with a set of three unique non- SMC subunits.
Where is the protein found in a chromosome?
Histone molecules are present throughout the length of the chromosomes as they take part in the first step of chromosome condensation. The nucleosome containing the histone core gets repeated every 200 base pairs in the chromatin fibre. As a result, the histone proteins are present throughout the length of the double stranded DNA.
On the other hand, the two types of condensin proteins that are present in the eukaryotic cells are found at different locations in a cell. Condensin I protein is usually found in the cytoplasm throughout the cell cycle expect during the mitosis or meiosis. When the nuclear membrane breaks down during prophase, these condensin I proteins are translocation into the nuclear space, to the chromosomes.
Whereas, condensin II proteins are always present inside the nucleus and take part in the early stages of chromosomal condensation.
What is the role of proteins in a chromosome?
Histone and non- histone proteins are responsible for helping the double stranded DNA to undergo supercoiling.
- The eight histone proteins, a pair each of H2A, H2B, H3 and H4, come together to form an octameric core upon which the double stranded DNA winds around the histone core 1.65 times. This arrangement is called a nucleosome and it forms the basic unit of chromosomal packing.
- H1 histone protein is present outside the nucleosome and is responsible for protecting the linker DNA present in between two nucleosomes.
- Nucleosomes compact the size of the DNA to the one- third of its original size.
- The histone proteins bind to the DNA with the help of 142 hydrogen bonds, along with several hydrophobic interactions and salt linkages.
- The amino acids present in the histone proteins form hydrogen bonds with the phosphate backbone of the DNA.
- Histones are rich in amino acids with basic side chains of 11 to 37 amino acids, containing residues such as lysine and arginine. When the histone proteins interact with the DNA, the positive charge of the side chains neutralizes the negative charge on of the phosphate of the DNA.
- The side chains of core proteins can also interact with those of the neighbouring nucleosomes to further stabilize the condensed structure.
- Histones also take part in gene regulation by acetylation and methylation of the side chains of the core protein.
- Acetylation of the amino acids residues of the histone side chain allows the DNA packed on the nucleosomes to loosen up for transcription. Methylation of the same allows the nucleosome to become tightly packed so that the DNA segment becomes transcriptionally inactive.
- For example, acetyltransferases enzyme attaches an acetyl group to the lysine residues present in the side chains of histone. This leads to destabilization of the DNA wound on the core protein, so that the bonds between them get weak. The loosely packed DNA then becomes free to undergo transcription.
- Chromosomes are highly condensed form of DNAs, that are super coiled with the help of some DNA binding proteins. So, chromosomes are composed of both nucleotides and proteins.
- Condensin I is present in the cytoplasm and so has to wait to get access to the chromosomes until the nuclear membrane to break down during meiosis. It is involved in the axial shortening of the chromosomes.
- In animal cells, condensin I responsible for keeping the kinetochore or the centromere of the chromosomes by preserving the heterochromatin in a highly condensed state.
- Together, both the condensin proteins collaborate in compaction of the chromosomes and get them ready to undergo separation by the metaphase of the cell division.
- The first stage of packaging during cell division occurs by coiling of the chromatin network into a 30 nm fibre during middle prophase. This is followed by supercoiling of the 30 nm coiled fibre into a 200 nm to 250 nm fibre by the end of the prophase.
Do human chromosomes contain protein?
Human beings are eukaryotic organisms, so their cells have chromosomes that are either super condensed or the loosely packed as chromatin fibre. Degree of compaction of the nuclear DNA is dependent on the phase of the cell cycle and keeps changing accordingly. As a result, histone proteins and non- histone proteins are always present in the cell, interacting with the DNA at some level or the other. Condensin I and II play the similar roles in human chromosomes as they do for all eukaryotic chromosomes.
Do bacterial chromosomes contain proteins?
Bacterial cells usually do not possess histones but archaea that are evolutionarily closer to eukaryotes, possess two types of histone that are similar to the eukaryotic H3 and H4 proteins. The archaeal histone protein form tetrameric core instead of octameric histone core that are present in the eukaryotic cells.
Many bacterial species and that of archaea, possess a condensin like complex that interact with their chromosome. While others such as gamma proteobacteria and Escherichia coli have different sets of chromosome binding proteins which are different from that of the eukaryotes.
The structural maintenance chromosomes or the SMCs are present in both the prokaryotic and the eukaryotic cells. In the bacterial cells, their function is analogous to that of the condensin proteins in the eukaryotic cells.
A common chromosomal protein that also takes part in the organisation of the bacterial chromosomes is nucleoid associated proteins or NAPs. They also affect the pattern of replication and transcription because in bacteria, the organisation of the chromosomes governs what kind of replication and transcription method it will undergoes.
NAPs also help the bacterial cells to cope up with different adverse environments by coating the chromosomal DNA and changing their shapes. This allows variable gene expression and the cell takes a quick reaction against the environmental changes.
Chromosomes are composed of double stranded DNA molecules and different protein cores. The DNA strands wound around the histone cores to form chromatin networks and are further condensed into chromosomes to prepare for cell division, by the condensin proteins.