35.2
Chromosome Duplication
The process of chromosome duplication during cell division requires genome-wide disruption and re-assembly of chromatin. The chromatin structure must…
The genetic material, the DNA, in a eukaryotic cell exists inside the nucleus as linear structures called the chromosomes.
Within the chromosomes, the double-stranded DNA primarily wraps around octameric nuclear proteins called histones to form nucleosomes. Several non-histone proteins also associate with the DNA, assisting in DNA packaging.
These nucleosomes and non-histone proteins arrange to form coils which further fold into loops, in an array, forming packed chromatin. The extent of chromatin compaction enables extremely long DNA to fit inside the nucleus. During M phase, chromatin further coils to form the condensed chromosomes.
Chromosome duplication involves duplicating the entire chromatin structure, so the DNA and chromatin-associated proteins are all duplicated. The production of histones is increased to generate the additional proteins required for packaging the newly synthesized DNA into chromosomes.
Replicating the exact chromatin structure of the duplicated chromosome is vital for gene regulation.
Within the chromosomes, the level of chromatin packaging is not always uniform. Chromosomes contain regions with tightly packed chromatin called heterochromatin and regions with loosely packed chromatin called euchromatin.
Heterochromatin DNA is inaccessible to the transcription machinery, and hence genes in those regions are not regularly transcribed. In contrast, the DNA within the euchromatin is more accessible, and thus genes in these regions can be transcribed.
This structural and functional significance of chromatin makes it crucial that the entire chromatin structure is reproduced accurately during chromosome duplication.
View the full transcript and gain access to JoVE Core videos
Q1: What is chromosome duplication and why does it occur?
Chromosome duplication is the process by which a cell creates an exact copy of each chromosome before division. This occurs during the S phase of the cell cycle to ensure that each daughter cell receives a complete set of genetic material. Duplication is essential for maintaining genetic continuity across generations of cells.
Q2: How does DNA replication relate to chromosome duplication?
DNA replication is the molecular mechanism underlying chromosome duplication. During replication, the DNA double helix unwinds and each strand serves as a template for synthesizing a new complementary strand. This produces two identical DNA molecules, which condense into duplicated chromosomes consisting of two sister chromatids joined at the centromere.
Q3: What are sister chromatids and how do they form?
Sister chromatids are two identical copies of a chromosome held together at a region called the centromere. They form during chromosome duplication when DNA replication creates two identical DNA molecules that remain attached. Sister chromatids separate during mitosis and cytokinesis, with each daughter cell receiving one copy of each chromosome.
Q4: When during the cell cycle does chromosome duplication happen?
Chromosome duplication occurs during the S phase (synthesis phase) of interphase, which is the period between cell divisions. The S phase follows the G1 phase and precedes the G2 phase. By the end of S phase, the cell has duplicated all its chromosomes, preparing for the subsequent mitotic division.
Q5: How does chromosome duplication ensure genetic material is preserved?
Chromosome duplication creates identical copies of each chromosome before cell division, guaranteeing that both daughter cells receive the same genetic information as the parent cell. This faithful copying mechanism maintains genetic continuity and ensures that essential genes and regulatory sequences are preserved through successive cell divisions.
Q6: What is the difference between chromosome duplication and cell division?
Chromosome duplication is the replication of genetic material that occurs before division, while cell division is the physical separation of duplicated chromosomes into two daughter cells. Duplication prepares the cell by creating copies; division distributes those copies. Both processes are essential for maintaining proper chromosome numbers in daughter cells.