Question

A human liver has patches of octaploid cells-they have eight sets of chromosomes. Explain how this might arise.

Short answer

Octaploid cells in the liver might have arisen due to the process of endoreduplication, where DNA replication occurred multiple times without cell division. The result is each cell having eight complete sets of chromosomes.

Step-by-step solution

Understanding Ploidy

Ploidy refers to the number of sets of chromosomes in a cell. Normally, human cells are diploid i.e., they have two sets of 23 chromosomes each, one set from each parent. But in this exercise, the liver cells are octaploid, which implies they contain eight sets of chromosomes.

Understanding DNA Replication

During cell division, DNA replication occurs in the S phase of the cell cycle, where every chromosome is duplicated so each daughter cell receives a complete set of chromosomes.

Understanding Endoreduplication

Endoreduplication is a process where DNA replication occurs without cell division (mitosis), leading to cells with multiple sets of chromosomes. This results in cells becoming polyploid.

Occurrence of Octaploidy

It's possible that in certain patches of liver cells, endoreduplication occurred three additional times without mitosis. This would transform the diploid cells (2 sets of chromosomes) into octaploid (8 sets of chromosomes) by doubling the chromosomes number three times. First endoreduplication would result in tetraploid (2\*2=4 sets); second one would produce hexaploid (2\*4=8 sets) and the third one would result in octaploid cells (2\*8=16 sets). If, following the second round of endoreduplication mitosis took place assuming it's incomplete and did not produce daughter cells, the cells would be octaploid, as the exercise describes.

Key concepts

Ploidy Levels

Ploidy levels refer to the number of sets of chromosomes present in a cell. These sets are fundamental components of genetic material. In human cells, the typical ploidy level is diploid, which means there are two sets of chromosomes: one set from each parent. Therefore, a diploid human cell contains a total of 46 chromosomes comprising 23 pairs. However, polyploid cells, like those found in certain parts of the liver, may have more than two sets of chromosomes.

• Diploid - 2 sets (2n)
• Tetraploid - 4 sets (4n)
• Hexaploid - 6 sets (6n)
• Octaploid - 8 sets (8n)

These levels can change due to processes like endoreduplication, leading to higher ploidy levels such as octaploid in liver cells.

Endoreduplication Process

Endoreduplication is a fascinating process where DNA replication takes place without subsequent cell division. This results in cells becoming polyploid, meaning they have multiple sets of chromosomes. Interestingly, during endoreduplication, cells skip the mitotic phase, where division usually takes place after the completion of DNA replication.
The process can be broken down into:

• The G1 phase: Cell growth and normal functions.
• The S phase: DNA replication occurs; every chromosome duplicates.
• Skipping of G2 and M phases: No mitosis or cytokinesis, resulting in polyploidy.

In human liver cells, endoreduplication allows for the existence of cells with more than the typical two sets of chromosomes, contributing to polyploidy like octaploidy.

Chromosome Sets in Liver Cells

Human liver cells are unique in that they often contain more than the usual diploid (2 sets of chromosomes) found in other cells. These cells can undergo endoreduplication, resulting in increased ploidy levels. This means some liver cells may become octaploid, possessing eight sets of chromosomes instead of just two.
Polyploid liver cells can provide certain advantages, such as enhanced metabolic capacity and the ability to respond to cellular damage. This polyploid nature showcases the liver's ability to adapt and function under various conditions, potentially supporting its role in detoxifying the body and managing various metabolic functions.

DNA Replication Stages

DNA replication is a critical part of the cell cycle, necessary for the creation of genetically identical daughter cells. It primarily occurs during the S phase of the cell cycle. The key stages of DNA replication include:

• Initiation: Unwinding of the DNA double helix.
• Elongation: Synthesis of the new DNA strands by DNA polymerase.
• Termination: Completion of the replication process, resulting in two identical DNA molecules.

Each chromosome in the cell is duplicated, ensuring that each daughter cell receives an exact copy of the genome. The precise regulation of DNA replication is vital for cellular function and genetic stability. In the context of endoreduplication in liver cells, DNA replication occurs without subsequent cell division, leading to polyploid cells with increased sets of chromosomes.