DAT Destroyer | OAT Destroyer
Many students preparing for the DAT struggle with the difference between chromosomes, chromatids, and sister chromatids. This topic appears frequently in the Biology section, and confusion often leads to missed questions. This guide provides a clear, student-friendly explanation with a simple table and stage-by-stage breakdown to help you master this concept before test day.
To understand how this topic fits into the overall structure of the exam, you may also find it helpful to review:
How Hard Is the DAT? A 2026 Difficulty Guide for Students
https://orgoman.com/blogs/dental-admissions-test-and-road-to-dental-school-acceptance/how-hard-is-the-dat-a-2026-difficulty-guide-for-students
What Is a Chromosome?
A chromosome is a single, continuous molecule of DNA. In humans, somatic cells contain 46 chromosomes arranged in 23 pairs. These chromosomes carry the genetic information needed for growth, cell function, and inheritance.
A chromosome can exist in two forms depending on the stage of the cell cycle:
-
Single Chromosome (Unreplicated)
A single DNA molecule before replication.
Example: In G1 phase, humans have 46 chromosomes and 46 chromatids. -
Duplicated Chromosome (Replicated)
After DNA replication during S phase, a chromosome consists of two identical sister chromatids attached at the centromere.
Example: In G2 phase, humans have 46 chromosomes but 92 chromatids.
The important point is that chromosome number does not change after replication. Chromatids double, but chromosomes remain the same until the chromatids separate.
What Is a Chromatid?
A chromatid is one copy of a duplicated chromosome. When DNA replicates, each chromosome becomes two identical sister chromatids.
A chromatid can be:
-
a single chromatid (before replication), or
-
one half of a duplicated sister chromatid pair (after replication)
Chromosome number is determined by the number of centromeres, not the amount of DNA. This distinction eliminates much of the confusion students experience on DAT questions.
Sister Chromatids vs. Chromosomes: The Numbering Rules
DAT questions often involve counting chromosomes and chromatids. These two rules make the process straightforward:
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Chromosomes = number of centromeres
-
Chromatids = total number of DNA copies (attached or separate)
Using these rules prevents almost all chromosome-counting errors.
Chromosome and Chromatid Counts by Cell Cycle Stage
Below is a reliable reference table for a human somatic cell (2n = 46):
| Stage of Cell Cycle | Chromosomes | Chromatids | Ploidy |
|---|---|---|---|
| G1 | 46 | 46 | Diploid |
| S Phase (after replication) | 46 | 92 | Diploid |
| G2 | 46 | 92 | Diploid |
| After Meiosis I | 23 | 46 | Haploid |
| After Meiosis II | 23 | 23 | Haploid |
Memorizing this table will help you handle nearly all chromosome and chromatid questions on the exam.
Why Students Answer These Questions Incorrectly
Three common misunderstandings lead to lost points:
-
Thinking that more DNA means more chromosomes
Chromosome number stays the same until sister chromatids separate. -
Confusing mitosis with meiosis
Mitosis maintains chromosome number.
Meiosis reduces chromosome number by half during Meiosis I. -
Not knowing when ploidy changes
Ploidy changes when homologous chromosomes separate in Meiosis I.
Understanding these points helps avoid most errors on this topic.
DAT Practice Question
A human cell (2n = 46) has just completed S phase. Which of the following correctly describes the number of chromosomes and chromatids present in this cell?
a. 23 chromosomes and 46 chromatids
b. 46 chromosomes and 46 chromatids
c. 46 chromosomes and 92 chromatids
d. 92 chromosomes and 92 chromatids
e. 92 chromosomes and 46 chromatids
Correct Answer: c
During S phase, each chromosome replicates into a pair of sister chromatids. The cell now has 46 chromosomes and 92 chromatids because the chromatids remain joined at their centromeres.
For a broader overview of the DAT structure, format, and section breakdown, you may also want to read:
What Is the DAT Exam?
https://orgoman.com/blogs/dental-admissions-test-and-road-to-dental-school-acceptance/what-is-the-dat-understanding-the-dental-admission-test-dat-for-students
Strengthen Your DAT Biology Knowledge
A clear understanding of chromosome behavior, chromatid structure, and cell cycle transitions can make a significant difference on test day. If you want to deepen your biology foundation and practice high-yield genetics and cell cycle questions, the following resources can help:
DAT Destroyer
A comprehensive set of high-yield practice problems covering Biology, General Chemistry, Organic Chemistry, Quantitative Reasoning, and Perceptual Ability.
https://orgoman.com/products/dat-destroyer
Dynamite Biology Review
A focused review written to help students quickly strengthen their understanding of the most frequently tested DAT biology concepts.
https://orgoman.com/products/dynamite-biology-review
Download Dr. Romano’s Free Biology Review
Students can download Dr. Romano’s full Biology Review for free in the DAT Destroyer Study Group on Facebook. The file is located in the Files section.
https://www.facebook.com/share/g/1BWoLC3JYq/
DAT & OAT Classes (Online and In-Person)
Live classes taught by Dr. Jim Romano, available both online and in person at Snug Harbor. These sessions provide guided review, problem-solving strategies, and structured support.
https://orgoman.com/pages/dat-classes