Problem 4 - Biology Today and Tomorrow with Physiology

Question

After DNA replication, how many chromatids does a chromosome have?

Step-by-Step Solution

Answer

After DNA replication, each chromosome has two chromatids.

Step 1: Understand the Concept

Chromosomes are structures within cells that contain DNA. Each chromosome contains two identical halves connected by a centromere, each half being a chromatid. During the cell cycle, cells undergo processes like DNA replication where each chromosome duplicates.

Step 2: Recognize DNA Replication

DNA replication is the process by which a cell makes an identical copy of its DNA. This occurs during the S phase of the cell cycle so that each daughter cell will have a complete set of chromosomes after cell division.

Step 3: Chromatids Doubling

After replication, each original chromatid is duplicated, resulting in two identical sister chromatids per chromosome. Therefore, each chromosome that originally had one chromatid will now have two chromatids after replication.

Step 4: Answer the Question

Since each chromosome was doubled during replication, post-replication, each chromosome has two chromatids instead of one.

Key Concepts

Chromatids

Chromatids are the two identical halves of a duplicated chromosome. Before DNA replication, a chromosome is composed of one chromatid – a single DNA molecule. Once replication occurs, this original chromatid is duplicated, resulting in two sister chromatids. These chromatids are genetically identical because they were created as precise copies of each other during replication.

• Sister chromatids are connected by a region called the centromere.
• Both chromatids contain the same genes in the same order since they are copies.

The connection between sister chromatids is crucial as it ensures proper segregation during cell division. The presence of chromatids ensures that each new cell will receive an exact copy of the DNA.

Chromosomes

Chromosomes are structures within cells that hold our DNA neatly packaged. This packaging allows for efficient distribution during cell division. Each human cell usually contains 23 pairs of chromosomes, for a total of 46. This number varies across different species.

• Before replication, each chromosome consists of one chromatid.
• Following replication, chromosomes consist of two sister chromatids.

This doubling is important for maintaining the correct number of chromosomes in daughter cells after cell division. Chromosomes ensure that genetic information is accurately transmitted and expressed in the offspring.

Cell Cycle

The cell cycle is a series of stages that cells go through as they grow and divide. It includes phases such as G1, S, G2, and M. These phases ensure that cells grow properly and replicate their DNA accurately before division.

• G1 Phase: The cell grows and conducts normal functions.
• S Phase: DNA replication occurs.
• G2 Phase: The cell prepares for division.
• M Phase: Mitosis and cytokinesis happen, resulting in two new cells.

The efficient progression through each of these phases is vital for cellular health and function, as errors can result in genetic abnormalities.

S Phase

The S phase, short for Synthesis phase, is a part of the cell cycle dedicated to DNA replication. During this phase, each chromosome is replicated to form two sister chromatids. It ensures that each resulting daughter cell will receive the full complement of DNA after cell division.

• DNA replication occurs, doubling the genetic material.
• This phase follows the G1 phase and precedes the G2 phase.

Proper DNA replication during the S phase is crucial for genetic stability, as copying errors can lead to mutations, which might result in disease. Therefore, the S phase is a critical period of the cell cycle that sets the stage for proper cellular reproduction.