Unless you have been living under a rock, you would know that the answer to this question is NO. A number of key experiments have shown that in many cell types mitosis removes chromosomes from one set and adds them to the other. If this is true, then we should never see cells with both copies of a chromosome (i.e., diploid), right? Wrong! There are numerous examples of mitotic cells with two copies of chromosomes. These results cause serious problems for our understanding of how mitotic cells work, as they imply there must be another mechanism to produce diploidy in mitotically active cell types. In this blog post, we will explore some of these experiments and what they mean for our understanding of mitotic cell biology.
Are Mitotic Cells Always Diploid?
A mitotic cell is a cell that has entered the cell cycle and is in the process of dividing. During the mitotic stage, a cell has one copy of its chromosomes and therefore, only one set of chromosomes. When a cell divides, each daughter cell receives one set of chromosomes from the parent.
When Is A Cell Diploid?
When we talk about a “diploid cell”, we are describing a cell with two full copies of the genome. These cells are essential for sexual reproduction and are also common in somatic tissues. To understand why mitotic cells are always supposed to be haploid, we need to think about how the cell cycles through mitosis and meiosis. Understandably, these processes are conserved across most eukaryotes. All eukaryotic cells need to duplicate their genome during the S phase so that each daughter cell has one complete genome.
Prokaryotic Organelles In Eukaryotic Cells
1. Meiotic Genome
The meiotic genome is the set of chromosomes that are replicated in meiosis I. Since there are four copies of each chromosome, each daughter cell has four complete genomes. This explains why diploid cells always divide by mitosis and why haploid cells always divide by meiosis.
2. Prokaryotic Organelles in Eukaryotic Cells
In addition to replicating their own genome, eukaryotes have a number of prokaryotic organelles that help with important processes like DNA replication and repair. These organelles include:
3. Ribosomes
Ribosomes are the cellular factories that produce proteins.
4. Endoplasmic Reticulum (ER)
The ER is a membrane-bound compartment that contains ribosomes and other organelles. The ER can also store proteins in “troughs” called cisternae.
5. Golgi Apparatus
The Golgi apparatus is a membrane-bound compartment that produces enzymes and other molecules needed to build and maintain the cell. It also produces secretory vesicles, which contain molecules like hormones and neurotransmitters.
6. Lysosomes & Peroxisomes (LPS/POPS)
Lysosomes are organelles that digest cellular debris, while peroxisomes are organelles responsible for lipid metabolism.
7. Lysosomes
Lysosomes are membrane-bound vesicles that can destroy or digest pathogens and damaged proteins. They also contain digestive enzymes to break down proteins and lipids for energy.
8. Endomembrane System (EM)
The endomembrane system is a membrane-bound compartment that delivers signals from the Golgi apparatus to the ER. It also transports vesicles between the ER and Golgi apparatus.
9. Mitochondria
Mitochondria are organelles that have their own DNA, ribosomes, and a number of other organelles inside them. They produce ATP through oxidative phosphorylation, which is the process by which cells convert adenosine triphosphate (ATP) into adenosine diphosphate (ADP). This happens in the mitochondrial matrix, which contains double membranes made up of inner membranes of stacked lipids and outer membranes stacked to build membranes and other structures.
10. Plasma Membrane
The plasma membrane is a lipid bilayer that provides a barrier for the cellular interior. It also contains receptors for signals from the outside world and can be modified by proteins, many of which come from prokaryotic organelles, to allow certain molecules in or out of the cell.
Chromosome Fusion In Mitosis
- The centrosome (CEN) is a spindle pole that sits in the middle of the cell. The CEN is responsible for pulling the chromosomes together and ensuring that they are evenly spaced during mitosis.
- Chromosomes must be pulled together under tension so that the centrosome can pull them toward each other. This tension is created by actin filaments pulled from the microtubule network and anchored to the CEN.
- Once the chromosomes are pulled together, they begin to move towards and become condensed in the middle of the cell (centromere). This process is called metaphase. The two daughter nuclei begin to separate from each other as they move toward their new positions in vivo.
- The two daughter nuclei begin to separate from each other as they move toward their new positions in vivo.
- During prophase, the chromosomes condense and the nuclear envelope breaks down, allowing the chromatin to become accessible to enzymes that are responsible for DNA replication and transcription (DNA polymerase and RNA polymerase).
- Prophase is followed by metaphase in which the chromosomes line up at the equator of the cell and then begin moving towards each other as they split into two cells. Two daughter nuclei form during metaphase. The process is called mitosis.
- Chromosomes now move into anaphase. In anaphase, the two chromatids split up and move toward opposite poles of the cell (anaphase B). The centrosome pulls the chromosomes together again and they begin to move toward their new positions in vivo.
- During telophase, the daughter nuclei reach their new locations and begin to condense once again. The nuclear envelope reforms as chromatin are removed from it, allowing transcription and replication to continue for mitosis to proceed.
- There are two types of anaphase: prometaphase and metaphase. In prometaphase, the sister chromatids begin to separate from each other and move towards opposite poles of the cell. During metaphase, they come together at a centromere, where they will eventually be pulled apart by microtubules during mitosis.
- The centrosome is responsible for pulling the chromosomes together under tension so that they can be pulled toward each other by microtubules. Microtubules form from pairs of end-to-end actin filaments and are embedded in a network of microfilaments called saturation of the nuclear envelope, which allows for transcription and DNA replication of the DNA.
Summary
Mitotic cells can be either haploid or diploid, depending on the type of mitosis they are in. Prokaryotic organelles in eukaryotic cells can also be diploid, due to the fusion of two chromosomes. Chromosomes can also be lost or fused during mitosis, with chromosome loss being the most commonly observed event. These events can be a useful way to study mitotic cell biology, as they allow us to determine how chromosomes are distributed between two cells.
