IB DP Biology- D2.1 Cell and nuclear division - IB Style Questions For SL Paper 2 - FA 2025

Answer:

1. Chromosomes condense through supercoiling.
2. Nuclear membrane breaks down.
3. Spindle fibers (microtubules) form and grow from centrosomes.
4. Spindle fibers attach to chromosomes at the centromeres.
5. Chromosomes align at the cell’s equator (metaphase plate).
6. Centromeres divide, separating sister chromatids.
7. Sister chromatids (now chromosomes) move to opposite poles.
8. Nuclear membranes reform around chromosomes at each pole.

Explanation:

Mitosis is the process of nuclear division that ensures each daughter cell receives an identical set of chromosomes. It occurs in four phases:

• Prophase: Chromosomes condense (supercoiling) to become visible, and the nuclear membrane disintegrates. Spindle fibers begin forming from centrosomes.
• Metaphase: Spindle fibers attach to centromeres via kinetochores, aligning chromosomes at the equator for equal distribution.
• Anaphase: Centromeres split, and motor proteins pull sister chromatids to opposite poles, ensuring each cell gets one copy of each chromosome.
• Telophase: Chromatids (now chromosomes) reach the poles, and new nuclear membranes form, completing the division of genetic material before cytokinesis splits the cytoplasm.

This sequence maintains genetic continuity in somatic cells for growth and repair.

Answer:

1. DNA replication occurs semi-conservatively.
2. DNA unwinds (helicase breaks hydrogen bonds, separating strands).
3. New DNA strands are synthesized by DNA polymerase using complementary base pairing.
4. DNA is transcribed into RNA (mRNA) by RNA polymerase.
5. Introns are removed from mRNA, and it is exported to the cytoplasm.

Explanation:

Interphase is the preparatory phase of the cell cycle, consisting of G1, S, and G2 stages, where the nucleus is active:

• DNA Replication (S Phase): The double helix unwinds via helicase, breaking hydrogen bonds between base pairs (A-T, C-G). DNA polymerase synthesizes new strands using the original strands as templates (semi-conservative replication), ensuring each daughter cell gets an identical genome. Replication is continuous on the leading strand and discontinuous (Okazaki fragments) on the lagging strand.
• Transcription (Throughout Interphase): Specific genes are transcribed into mRNA by RNA polymerase. Complementary base pairing (A-U, C-G) copies the gene sequence. Pre-mRNA is processed by removing introns (non-coding regions), and mature mRNA exits the nucleus via nuclear pores for protein synthesis in the cytoplasm.
• Cell Growth and Checks (G1/G2): While not directly nuclear processes, the nucleus oversees protein synthesis and organelle duplication, preparing for mitosis.

These processes ensure the cell is ready for division, doubling its genetic material and producing necessary proteins.

Answer:

1. The Y chromosome contains a gene (SRY) that triggers testes development.
2. Developing testes secrete testosterone, which promotes male genitalia formation.

Explanation:

In human embryos, sex is determined by the presence or absence of the Y chromosome:

• SRY Gene: The Y chromosome carries the SRY (Sex-determining Region Y) gene. Around 6-7 weeks of embryonic development, SRY expression initiates the differentiation of gonadal tissue into testes rather than ovaries (which form in the absence of SRY).
• Hormonal Influence: The testes produce testosterone, a hormone that drives the development of male internal (e.g., vas deferens) and external (e.g., penis, scrotum) genitalia. Without testosterone (in XX embryos), female structures develop instead.

This genetic switch illustrates how a single chromosome can dictate developmental pathways via gene expression and hormonal signaling.