The chromosomes are located in the nucleus of the cell. Both plant cells and animal cells have chromosomes within their nucleus, and every chromosome is comprised of a single molecule of deoxyribonucleic acid and proteins. Let’s examine the role of chromosomes within the nucleus and the cell at large, and see how chromosomes are replicated during the process of cellular division.
What Is The Nucleus Of The Cell?
The nucleus of a cell is often said to be the “brain” of the cell. The nucleus is a membrane-bound organelle which functions to control the reproduction and growth of the cell, as well as contain the hereditary/genetic information for the cell. The nucleus is the largest organelle in the cell, and it is the command center of all eukaryotic cells. Prokaryotic cells don’t have a “true” nucleus, although they do have a region where the hereditary information lies.
The contents of the nucleus are contained in a structure referred to as the nuclear envelope, a double-membrane structure that is selectively permeable, meaning it only allows certain substances to pass in and out of it. Much like the membrane of the cell as a whole the nuclear envelope is made out of a phospholipid bilayer, two layers that have a phosphate head and a lipid tail linked together and facing opposite directions. The structure of the nuclear envelope helps the nucleus maintain its shape and assists the nucleus in controlling which molecules move in and out of it. Molecules which are permitted to enter the nucleus enter through small holes in the membrane referred to as nuclear pores. The nuclear envelope is linked to the endoplasmic reticulum in a way that the internal compartment of the nuclear envelope forms a contiguous region with the inner portion (the lumen) of the endoplasmic reticulum.
The nucleus is where the chromosomes are held, and the chromosomes are composed of DNA, which is often said to be the “blueprint for life” as it contains the necessary information for the development and reproduction of cells. Cells can be considered to have two different states a default “resting” state, and an active “dividing” state. When the cell isn’t dividing the chromosomes are bundled up, organized in structures dubbed chromatin. This means that while people tend to think of the chromosomes in the nucleus as individual chromosomes floating around, they are actually stitched together in complex structures.
Beyond the chromosomes, the nucleus also contains other structures and components. The cell nucleus also contains the nucleoplasm and the nucleolus.
The nucleoplasm is similar to the cytoplasm within the cell, a jelly-like substance composed mainly out of water, organic molecules, enzymes, and dissolved salts. The nucleoplasm is sometimes referred to as the karyoplasm. The chromosomes and nucleolus are enveloped by the nucleoplasm, and the nucleoplasm helps protect and cushion these structures. The nucleoplasm also assists the nucleus in maintaining its shape, much like how the cytoplasm and cytoskeleton do for the rest of the cell. The nucleoplasm also helps the nucleotides and enzymes be transported around the nucleus and through the nucleus pores.
Inside the nucleus is a smaller organelle referred to as the nucleolus. The nucleolus is a large bundle of proteins and RNA, and unlike the nucleus, it is not membrane-bound. The nucleus also possesses chromosome parts with genes controlling the synthesis of ribosomes in them. These chromosomes parts are called nucleolar organizers. The function of the nucleolus is to create ribosomes by assembling and transcribing the subunits which will come together to create a complete ribosome during the process of protein synthesis.
Structure And Function Of The Chromosome
Chromosomes are aggregated collections of genes, made out of tightly wound packets of chromatin, which are themselves composed out of proteins and DNA. When chromosomes are paired together, when there is one chromosome from a father and another chromosome from a mother, these are called homologous chromosomes.
Chromosomes can be either duplicated or not duplicated. None duplicated chromosomes are comprised of a centromere, a central structure which links the two arms of the chromosome together. Not duplicated chromosomes are single-stranded. The long arm of the non-duplicated chromosome is referred to as the Q-arm, while the short arm is dubbed the P-arm. At the ends of the chromosomes are regions referred to as telomeres, which are made up of non-coding DNA sequences.
Before a cell divides, the chromosomes are duplicated through the process of DNA replication, which lets the two daughter cells maintain the correct number of chromosomes after the cell has divided. Duplicated chromosomes are made out of two chromosomes identical to one another, joined together at the centromeres, and called sister chromatids. The sister chromatids will stay linked until the final phase of the cellular division process when they will be pulled apart by structures called spindle fibers. After the separation of the chromatids is complete the individual chromosomes are referred to as daughter chromosomes.
Cellular Division And Chromosomes
When cells are divided the correct number of chromosomes must be maintained. In the process of mitosis, the reproduction of nonsex cells, the chromosomes have to be distributed among only two daughter cells. In contrast, meiosis is the reproduction of sex cells and these chromosomes have to be distributed across four daughter cells.
During the course of cellular division, the chromosomes are moved around by a structure known as the spindle apparatus and its accompanying spindle fibers. The spindle apparatus has things known as microtubules and motor proteins which are utilized to pull the chromosomes apart and shift them to the correct positions. The separation and organization of chromosomes must be carried out perfectly because every cell that will be created from the division process must have the correct number of chromosomes. If an error occurs in cell division, the individual in question may have an unbalanced number of chromosomes, either too few chromosomes or too many chromosomes. An improper number of chromosomes frequently results and developmental disabilities, birth defects, and sometimes death.
Likewise, mutations to the chromosomes usually have harmful consequences like birth defects and developmental disabilities. This can happen as a result of either improper cellular division, or exposure to radiation/mutagenic chemicals. Improper duplication of proteins can lead to chromosome structural changes, and the vast majority of these mutations are harmful. Abnormal numbers of chromosomes typically result due to the failure of homologous chromosomes to be properly separated during cellular division.
Chromosomes and DNA are necessary for protein production, a critical cell process. The genes found in the DNA code for the production of specific proteins, and during the process of protein synthesis the DNA is unwound (or unzipped) and the DNA’s information transcribed into another molecule known as RNA. This transcription carried by the RNA is then translated and brought to ribosomes where the information is used to make proteins.
Chromosomes During The Phases Of Cellular Division
During the mitosis process, the chromosomes are copied to create sister chromatids during Interphase. Meanwhile, in Prophase, the chromosomes begin to condense. The process of chromosome condensation makes the chromosomes easier to separate later during the actual process of cellular division, or cytokinesis. During the late Prophase, sometimes referred to as Pro-metaphase, the mitotic spindle stretches out and captures the chromosomes within it. The microtubules of the mitotic spindle are fused to the chromosomes at a region referred to as the kinetochore, a chunk of protein found at the centromeres of the chromatids.
In metaphase, the chromosomes are maneuvered by the mitotic spindle to the middle of the cell, lining up at a region referred to as the metaphase plate. Microtubules from opposite poles of the cell are each attached to two kinetochore’s of each chromosome, preparing to split them apart in the next phase. During anaphase, the actual separation of the chromosomes happens, and the sister chromatids are pulled apart, dragged towards opposite ends of the cell by the mitotic spindle. This process ensures that the chromosomes are in their correct position when telophase occurs and the cell divides into two, with each new cell having the correct number of chromosomes. After division, the chromosomes will de-condense and return to their regular string-like form.
Meiosis, the process of cellular division that produces sex cells, creates four daughter cells instead of two daughter cells. This means that the chromosome number will be half the number of chromosomes in the parent cell, contrasted with mitosis where the chromosome number in the daughter cells remains the same as the parent cell. These daughter cells with half the number of chromosomes are referred to as haploid cells.
During meiosis, a process of genetic recombination, or random swapping, occurs in the chromosomes. This happens during Prophase one of meiosis and it is referred to as crossing over. Random pieces of one chromosome are exchanged with the equivalent areas of another chromosome, creating a new unique DNA combination. Another difference between mitosis and meiosis is that while in Metaphase of mitosis individual chromosomes line up on the metaphase plate, pairs of chromosomes line up on the metaphase plate in Metaphase one of meiosis. Finally, meiosis actually has two rounds of cellular division compared to the one round of the date vision and mitosis. In meiosis Anaphase one, the sister chromatids are moves to the same pole, while in meiosis Anaphase two the chromatids are pulled to opposite poles.