Cell cycle and Cell divisions

What are the distinct stages of the cell cycle?

G1 (growth), S (DNA replication), G2 (preparation for division), and M phase (mitosis and cytokinesis). Some cells enter G0, a non-dividing state.

Why do the G1 and G2 phases exist?

G1 allows the cell to grow and perform normal functions; G2 allows further growth, preparation for mitosis, and checking of replicated DNA.

Where are actively cycling cells found in multicellular animals?

They are mainly found in tissues requiring continual growth or replacement, such as bone marrow, intestinal lining, skin, hair follicles, and tissues undergoing repair.

What happens during G1?

The cell grows, performs normal functions, produces proteins and organelles, and prepares for DNA replication.

What happens during S phase?

The cell replicates its DNA so each chromosome consists of two sister chromatids.

What happens during G2?

The cell continues growing, prepares for mitosis, and checks the replicated DNA.

What happens during M phase?

Mitosis divides the nucleus, followed by cytokinesis, which divides the cytoplasm.

What is G0?

A non-dividing state in which a cell performs specialized functions but is not actively progressing through the cell cycle.

What is the purpose of cell division?

Cell division allows organisms to grow, replace damaged or dead cells, repair tissues, and reproduce in some organisms.

Why is it dangerous for a cell to become too large?

As a cell grows, its surface-area-to-volume ratio decreases, making nutrient uptake, gas exchange, and waste removal less efficient.

What is the main role of cell-cycle checkpoints?

Checkpoints monitor whether a cell is ready to proceed and can stop the cell cycle if DNA is damaged, replication is incomplete, chromosomes are improperly attached, or conditions are unsuitable.

What is positive regulation of the cell cycle?

Positive regulation uses proteins and signals that promote progression through the cell cycle.

What is negative regulation of the cell cycle?

Negative regulation uses proteins and signals that stop or slow cell-cycle progression when problems are detected.

What proteins are involved in positive cell-cycle regulation?

Cyclins and cyclin-dependent kinases (CDKs). Cyclins activate CDKs, and active CDKs phosphorylate target proteins to promote cell-cycle progression.

What proteins are involved in negative cell-cycle regulation?

Tumor suppressor proteins such as p53 and Rb can inhibit cell-cycle progression when conditions are unsafe.

What happens if a checkpoint protein does not function properly?

Damaged or abnormal cells may continue dividing, allowing mutations and chromosome abnormalities to accumulate and potentially leading to cancer.

Why is p53 called the guardian of the genome?

p53 responds to DNA damage by stopping the cell cycle, promoting DNA repair, or triggering apoptosis if the damage cannot be repaired.

What is apoptosis?

Apoptosis is programmed cell death used to safely eliminate unnecessary, damaged, infected, or potentially dangerous cells.

Give examples of situations where cells undergo apoptosis.

Cells may undergo apoptosis when they have severe DNA damage, are no longer needed during development, are infected by certain viruses, become potentially cancerous, or pose a risk to the organism.

What are the stages of mitosis?

Prophase, metaphase, anaphase, and telophase, followed by cytokinesis.

What happens during prophase?

Chromosomes condense, the spindle forms, and the nuclear envelope breaks down.

What happens during metaphase?

Chromosomes line up at the cell's equator.

What happens during anaphase?

Sister chromatids separate and move toward opposite poles.

What happens during telophase?

New nuclear envelopes form around the chromosomes at opposite poles.

What does mitosis ensure?

Mitosis ensures that daughter cells receive essentially the same genetic information and chromosome number as the parent cell.

What are the main differences between mitosis and meiosis?

Mitosis has one division and produces two genetically similar cells; meiosis has two divisions and produces four genetically different haploid cells.

What is the purpose of meiosis?

Meiosis produces haploid cells and creates genetic variation through crossing over and independent assortment.

How can you identify metaphase in a cell diagram?

Chromosomes are lined up single-file at the middle of the cell.

How can you identify anaphase in a cell diagram?

Chromosomes or chromatids are moving toward opposite poles.

How can you identify prophase I of meiosis?

Homologous chromosomes pair together and crossing over can occur.

How can you identify metaphase I of meiosis?

Homologous chromosome pairs line up together at the middle of the cell.

How can you identify anaphase I of meiosis?

Homologous chromosomes separate while sister chromatids remain attached.

How can you identify anaphase II?

Sister chromatids separate and move toward opposite poles.

Why is meiosis I called reductional division?

Because homologous chromosomes separate, reducing the chromosome number from diploid (2n) to haploid (n).

Why is meiosis II called equational division?

Because sister chromatids separate while the chromosome number remains haploid.

What are homologous chromosomes?

Matching chromosomes, one inherited from each parent, that contain the same genes at the same loci but may contain different alleles.

What is synapsis?

Synapsis is the pairing of homologous chromosomes during prophase I of meiosis.

What is the synaptonemal complex?

A protein structure that helps hold homologous chromosomes together during prophase I, allowing recombination between non-sister chromatids.

What is crossing over?

The exchange of corresponding DNA segments between non-sister chromatids of homologous chromosomes during prophase I.

What are chiasmata?

The visible points where homologous chromosomes remain connected after crossing over.

How does crossing over create novel combinations of alleles?

Non-sister chromatids exchange DNA segments, producing chromosomes with new combinations of alleles.

What are linked genes?

Genes located close together on the same chromosome that tend to be inherited together.

How does distance between genes affect linkage?

The closer two genes are, the less likely crossing over will occur between them and the more likely they are to be inherited together.

What happens when genes are far apart on a chromosome?

Crossing over is more likely to occur between them, so they are less likely to be inherited together.

What are the major sources of genetic variation associated with meiosis?

Crossing over, independent assortment of homologous chromosomes, and random fertilization.

What is independent assortment?

The random orientation of homologous chromosome pairs during metaphase I, which creates different combinations of maternal and paternal chromosomes in gametes.

What is aneuploidy?

A condition in which a cell has an abnormal number of chromosomes.

What is nondisjunction?

The failure of homologous chromosomes or sister chromatids to separate properly during meiosis.

What happens during nondisjunction in meiosis I?

Homologous chromosomes fail to separate, producing gametes with abnormal chromosome numbers.

What happens during nondisjunction in meiosis II?

Sister chromatids fail to separate, producing some gametes with abnormal chromosome numbers.

What is the relationship between maternal age and Down syndrome risk?

The risk of a child having Down syndrome (trisomy 21) increases with maternal age, partly because aging oocytes have an increased risk of meiotic nondisjunction.

What causes Down syndrome?

Down syndrome is usually caused by an extra copy of chromosome 21, resulting in trisomy 21.

What does meiosis produce in animals?

Meiosis produces haploid gametes such as sperm and eggs.

What does mitosis produce in animals?

Mitosis produces diploid body cells used for growth, repair, and maintenance.

What does meiosis produce in plants?

Meiosis produces haploid spores.

What does mitosis produce in plants?

Mitosis produces cells for growth of both the haploid gametophyte and diploid sporophyte stages.

What does meiosis produce in fungi?

Meiosis generally produces haploid spores.

What does mitosis produce in fungi?

Mitosis produces additional cells during the largely haploid portions of the fungal life cycle.

What does meiosis produce in algae?

It varies by species; meiosis may produce haploid spores or haploid cells depending on the life cycle.

What does mitosis produce in algae?

Mitosis produces cells within either haploid or diploid stages, depending on the species and life cycle.