Chapter 12 – The Cell Cycle (BSC2010c)

A comprehensive set of 50 question-and-answer flashcards covering chromosome structure, phases of the cell cycle, mitotic mechanics, regulatory mechanisms, and cancer biology as presented in the lecture notes.

What are somatic cells and how many chromosomes do they contain in humans?

Somatic cells are all non-reproductive body cells and they contain 46 chromosomes (23 pairs).

Which human cells are called gametes and how many chromosomes do they carry?

Gametes are sperm or egg cells and each carries 23 chromosomes.

When a sperm and an egg fuse, how many total chromosomes does the resulting zygote have?

46 chromosomes.

During which main phase of the cell cycle does a somatic cell spend most of its life?

Interphase.

During which phase of the cell cycle does active nuclear division occur?

Mitosis.

Define a cell’s genome.

The complete set of an organism’s DNA, including all of its genes.

In what form is DNA found during interphase, and why is this form advantageous?

Chromatin – a relaxed, uncondensed structure that allows easy access for replication and transcription.

What is chromatin?

One long, linear DNA molecule associated with proteins, existing in an uncondensed form.

What structure on chromatin serves as an anchoring point during mitosis?

The centromere.

What are sister chromatids?

Two identical copies of a DNA molecule produced after replication and held together by cohesin proteins.

At what point do sister chromatids become called chromosomes?

After they separate during anaphase.

What proteins hold sister chromatids together and what is this attachment called?

Cohesin proteins; the attachment is called sister chromatid cohesion.

How many DNA molecules are present in a duplicated chromosome?

Two (the sister chromatids).

How does the bacterial genome differ from a eukaryotic one in structure?

Bacteria typically have a single, circular DNA molecule and do not form condensed visible chromosomes.

List the three sub-phases of interphase in order.

G1 phase, S phase, G2 phase.

What key event occurs during the S phase of interphase?

Replication of DNA.

Which two cellular structures make up a centrosome?

Two centrioles and surrounding pericentriolar material.

Name the five phases of mitosis in the correct sequence.

Prophase, Prometaphase, Metaphase, Anaphase, Telophase (followed by Cytokinesis).

What defining event characterizes prophase?

Condensation of chromatin into visible chromosomes and the beginning of spindle formation.

What major change to the nuclear envelope occurs in prometaphase?

The nuclear envelope fragments (breaks down).

Where do chromosomes align during metaphase, and what checkpoint occurs here?

At the metaphase plate; the metaphase checkpoint ensures proper kinetochore attachment.

Which enzyme cleaves cohesin to initiate anaphase?

Separase.

What happens to sister chromatids during anaphase?

They are pulled apart and move to opposite poles of the cell.

Describe two theories for chromosome movement along kinetochore microtubules in anaphase.

1) Pac-Man model: motor proteins at kinetochores walk chromosomes toward poles while microtubules depolymerize at the kinetochore end.
2) Reeling-in model: motor proteins at spindle poles pull in microtubules, depolymerizing them at the pole end.

What process physically divides the cytoplasm following mitosis?

Cytokinesis.

How does cytokinesis differ in animal and plant cells?

Animals use a contractile ring to form a cleavage furrow; plants form a cell plate from Golgi-derived vesicles.

What is the mitotic spindle and of what three main elements is it composed?

A structure of microtubules that segregates chromosomes; composed of centrosomes, spindle microtubules, and asters.

What are kinetochore microtubules?

Spindle fibers that attach to the kinetochore at each chromosome’s centromere and pull chromatids apart.

What are polar (non-kinetochore) microtubules and their role?

Microtubules that overlap at the cell center, pushing spindle poles apart and elongating the cell during anaphase.

From which cellular region do microtubules of the spindle originate?

The centrosome, also known as the microtubule-organizing center (MTOC).

Which type of reproduction is binary fission and in which organisms does it occur?

Asexual reproduction in prokaryotes and some single-celled eukaryotes.

What proteins regulate progression through the eukaryotic cell cycle?

Cyclins and cyclin-dependent kinases (CDKs).

What does MPF stand for and what checkpoint does it trigger?

Maturation (or M-phase) Promoting Factor; it triggers passage through the G2 checkpoint into M phase.

Name the three principal cell-cycle checkpoints.

G1 checkpoint, G2 checkpoint, M (spindle) checkpoint.

What decision is made at the G1 checkpoint?

Whether the cell proceeds to S phase or enters the non-dividing G0 state.

Define density-dependent inhibition.

An external physical cue where crowded cells stop dividing when they touch neighboring cells.

What is anchorage dependence?

Requirement that cells attach to a substrate or extracellular matrix to divide and survive.

How do cancer cells differ from normal cells regarding growth factors and checkpoints?

Cancer cells may produce their own growth factors, ignore external cues, and bypass cell cycle checkpoints, leading to uncontrolled division.

Differentiate between benign and malignant tumors.

Benign tumors remain localized and non-invasive; malignant tumors invade surrounding tissues and can metastasize.

What is metastasis?

The spread of cancer cells from their original site to distant tissues forming new tumors.

During which stage of the cell cycle are chromosomes counted as 92 in a human cell, and why?

During prophase or metaphase after DNA replication, because each duplicated chromosome contains two sister chromatids (92 chromatids counted as 92 chromatids but still 46 chromosomes by convention until separation).

After anaphase, how many chromosomes does each pole of a human cell contain?

46 chromosomes (because sister chromatids have separated and are now individual chromosomes).

By the end of cytokinesis, how many chromosomes does each daughter human cell contain?

46 chromosomes (identical to the parent).

What structural difference exists between animal and plant cells regarding spindle origination?

Animals use centrosomes with centrioles; plants build spindles from dispersed MTOCs without centrioles.

Explain the role of cyclin degradation in cell-cycle control.

Cyclin breakdown inactivates CDKs (e.g., MPF), ensuring that mitotic events occur only once per cycle and allowing exit from M phase.

Which external protein signal is essential for fibroblast division and what cell type produces it?

Platelet-derived growth factor (PDGF) produced by blood platelets.

What is the metaphase plate?

An imaginary plane equidistant between the spindle’s two poles where chromosomes align during metaphase.

Which microtubule behavior adds tubulin subunits to extend the spindle, and which removes them to shorten it?

Polymerization adds subunits (elongation); depolymerization removes subunits (shortening).

What are asters and when do they appear?

Radial arrays of short microtubules that extend from each centrosome during spindle formation in prophase.

Name two key similarities between mitosis in animal and plant cells.

Both proceed through the same five mitotic phases and produce two genetically identical daughter cells.