Question

A replicated chromosome is composed of a. two homologous chromosomes held together at the centromere. b. four sister chromatids held together at the centromere. c. two sister chromatids held together at the centromere. d. four homologous chromosomes held together at the centromere. e. one chromosome with a centromere.

Short answer

The correct answer is (c) two sister chromatids held together at the centromere.

Step-by-step solution

Understand the Terms

Homologous chromosomes are pairs of chromosomes, one from each parent, that have the same gene structure. Sister chromatids are identical copies of a single chromosome, resulted from replication, that are attached to each other at a point called a centromere.

Evaluate Options

Go through each option and evaluate whether it correctly describes a replicated chromosome, based on understanding of the terms.

Select Correct Answer

Option (c), 'two sister chromatids held together at the centromere', correctly describes the structure of a replicated chromosome.

Key concepts

Sister Chromatids

When a cell prepares to divide, it needs to ensure that each new daughter cell receives an exact copy of its DNA. This is where sister chromatids come into play. Sister chromatids are the two identical halves of a single replicated chromosome. They are created during the S phase of the cell cycle, when the DNA in a single chromosome is copied.
While they are identical, sister chromatids remain bound together until the cell divides. The structure of sister chromatids is crucial during the process of cell division because it guarantees that each daughter cell receives an accurate copy of the cell's genetic information.

• Identical: Sister chromatids are exact copies of one another.
• Held Together: They are linked at a specific point called the centromere.
• Separation: They will only separate during the later stages of cell division.

Centromere

The centromere plays a vital role in the life of a replicated chromosome. It is the region where the identical sister chromatids are tightly connected during the initial stages of cell division.
Located roughly in the middle of the chromosome, the centromere ensures proper alignment and segregation of the chromosomes during mitosis and meiosis.
Its primary functions are:

• Attachment: Holds the sister chromatids together.
• Conjugation: Connects to spindle fibers during cell division, which is key for pulling chromatids apart.
• Stability: Provides structural stability to the chromosome.

Understanding the centromere's role helps clarify why a replicated chromosome is defined by the joined structure of two sister chromatids. This connection is essential for maintaining the integrity and stability of genetic material throughout cell division.

Homologous Chromosomes

Homologous chromosomes are pairs of chromosomes that are similar but not identical. Each pair consists of one chromosome inherited from the mother and one from the father. While they carry the same set of genes, the specific alleles (variations of a gene) may differ between them.
These chromosomes are pivotal during sexual reproduction as they play a role in genetic diversity and crossovers.
Key points about homologous chromosomes:

• Pairing: They align with each other during meiosis.
• Genetic Variation: They exchange genetic material, which increases variability among offspring.
• Similarity: They are similar in size, shape, and gene structure but can have different genetic information.

The distinction between homologous chromosomes and sister chromatids is critical in genetics education. While sister chromatids are identical copies, homologous chromosomes represent genetic diversity and variation.