Problem 12 - Biology Concepts and Applications

Question

After DNA replication, a eukaryotic chromosome ________. a. consists of two sister chromatids b has a characteristic X shape c. is constricted at the centromere d. all of the above

Step-by-Step Solution

Answer

The answer is d. all of the above.

Step 1: Understanding the Question

Begin by carefully reading the question and all the options provided. It's important to identify what is being asked specifically about a eukaryotic chromosome after DNA replication.

Step 2: Recalling DNA Replication

Recall that DNA replication is a process that occurs during the S phase of the cell cycle. After replication, each chromosome consists of two identical copies, known as sister chromatids.

Step 3: Analyzing Option A

Option a states that after DNA replication, a chromosome consists of two sister chromatids. This statement is true as the chromatids are identical copies joined together.

Step 4: Analyzing Option B

Option b describes that after DNA replication, the chromosome has a characteristic X shape. When chromatids are paired and joined at the centromere, this X shape is observable.

Step 5: Analyzing Option C

Option c states that the chromosome is constricted at the centromere. This is true after replication, as the sister chromatids are joined at the centromere, forming the constricted region.

Step 6: Evaluating Option D

Option d states all of the above. Since options a, b, and c are all correct descriptions of the state of the chromosome after DNA replication, this option is also correct.

Key Concepts

Eukaryotic Chromosome

Eukaryotic chromosomes are structures found in the nucleus of eukaryotic cells. They carry genetic information in the form of DNA. Each chromosome is composed of DNA coiled around proteins called histones, which help package the DNA into a compact form.
The number of chromosomes in eukaryotic organisms varies across species. For example:

• Humans have 46 chromosomes.
• Fruit flies have 8 chromosomes.

During the life cycle of a cell, especially during cell division, these chromosomes undergo significant changes. Understanding these transformations is crucial for comprehending how cells replicate and divide.

Sister Chromatids

Sister chromatids are two identical halves of a duplicated chromosome. They form as a result of DNA replication during the S phase of the cell cycle. These chromatids are exact copies of each other because they replicate the same DNA sequence.
Once formed, sister chromatids are attached to each other at a region called the centromere. This attachment is particularly important during cell division:

• It helps ensure that each new cell receives an identical set of chromosomes.
• The centromere serves as a vital link for the sister chromatids until they are pulled apart in the later stages of cell division.

Visually, when connected, they might resemble an 'X' shape, a common depiction of chromosomes after replication.

Cell Cycle

The cell cycle is the ordered series of events that cells go through as they grow and divide. It consists of distinct phases, beginning with the G1 phase, moving through the S and G2 phases, and concluding with the M phase. Let's break it down:

• **G1 phase:** The cell grows and synthesizes proteins required for DNA replication.
• **S phase:** DNA replication occurs, resulting in the formation of sister chromatids for each chromosome.
• **G2 phase:** The cell continues to grow and prepares for mitosis.
• **M phase (Mitosis):** The division of the cell's nucleus and its contents, followed by cytokinesis, which divides the cytoplasm and cell membrane.

The successful completion of the cell cycle is essential for cell growth, tissue repair, and in some organisms, reproduction. Each phase must proceed in a well-orchestrated manner to maintain genetic stability in the offspring cells.