Question 1
A human skin cell is about to undergo mitosis. During which sub-stage of interphase does the cell copy all of its chromosomes?No clue? Show me the answer
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Method #1Direct ApproachStep 1: Identify the question focus
The question asks which sub-stage of interphase involves DNA replication (copying chromosomes). Interphase has three sub-stages: G₁, S, and G₂.
Step 2: Recall what happens in each sub-stage
G₁ is for cell growth and protein production. S phase is specifically named 'Synthesis' because this is when DNA is synthesised (replicated). G₂ is for continued growth and checking that DNA replication was accurate.
Step 3: Select the correct answer
DNA replication occurs during S (Synthesis) phase. The name itself is a clue — 'synthesis' refers to making new DNA copies. Each chromosome is replicated to form two identical sister chromatids joined at the centromere.
Method #2Process of EliminationStep 1: Identify what is being asked
We need to find the sub-stage of interphase where chromosome copying (DNA replication) occurs.
Step 2: Eliminate G₁
G₁ (Gap 1) is when the cell grows larger and produces proteins and organelles — DNA is not replicated here. Eliminate this option.
Step 3: Eliminate G₂
G₂ (Gap 2) occurs after DNA replication — it is when the cell checks for replication errors and continues growing. DNA replication has already happened by this point. Eliminate this option.
Step 4: Eliminate Prophase
Prophase is the first stage of mitosis, not a sub-stage of interphase. DNA replication must be complete before mitosis begins. Eliminate this option.
Step 5: Select the correct answer
S (Synthesis) phase is the correct answer. The 'S' literally stands for synthesis of DNA — this is when all chromosomes are replicated, producing sister chromatids.
Question 2
After DNA replication, a chromosome consists of two identical copies joined together. What is the correct term for each of these copies?No clue? Show me the answer
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Choose a solution method
Method #1Direct ApproachStep 1: Identify what the question describes
The question describes two identical copies of a chromosome that are joined together after DNA replication. We need to identify the correct biological term for these structures.
Step 2: Apply the definition
After DNA replication in S phase, each chromosome consists of two identical sister chromatids joined at the centromere. The word 'sister' indicates they are identical copies, and 'chromatid' refers to each individual strand.
Step 3: Confirm the answer
The correct term is sister chromatids. They remain joined until anaphase, when the centromere splits and each chromatid moves to opposite poles to become an individual chromosome in a daughter cell.
Method #2Process of EliminationStep 1: Identify what is being asked
We need to name the two identical joined copies of a chromosome that exist after DNA replication.
Step 2: Eliminate homologous chromosomes
Homologous chromosomes are pairs of chromosomes that carry the same genes but come from different parents (one maternal, one paternal). They are not identical copies — they may carry different versions of genes. Eliminate this option.
Step 3: Eliminate centromeres
The centromere is the joining region where sister chromatids are held together — it is a part of the chromosome structure, not the name for the copied strands themselves. Eliminate this option.
Step 4: Eliminate spindle fibres
Spindle fibres are protein structures that move chromosomes during cell division. They are not DNA copies. Eliminate this option.
Step 5: Select the correct answer
Sister chromatids is the correct term — two identical DNA copies joined at the centromere, produced during DNA replication in S phase.
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