Bio Unit 2- Cell Cycle

Do all cells in the G₀ phase behave the same way?

No, not all G₀ cells are the same. Some remain in G₀ permanently, while others can re-enter the cell cycle or function continuously.

what are two cell types that remain in G₀ permanently for life?

• Heart Muscle (Cardiomyocytes)

• Brain Cells (Neurons)

What is the cellular behavior of Liver cells (hepatocytes) regarding the cell cycle?

They function normally in G₀ but can regenerate if they receive signals to go back into the cell cycle.

How do Lymphocytes (blood cells) interact with the cell cycle during an infection?

• They enter the cell cycle to replicate and attack a pathogen.

• After the pathogen is defeated, they go back to G₀.

What unique ability do Stem cells (Embryonic and Adult) possess?

They can differentiate into any cell type.

What three steps must stem cells follow to be useful in regenerative medicine?

They must divide, differentiate, and then stop dividing.

What field of medicine is made possible by cell-cycle control?

Regenerative medicine (e.g., using stem-cell trials to treat conditions like Parkinson's disease)

What are four reasons why cells divide instead of just growing larger?

• Growth and development

• Tissue repair and regeneration

• Production of specialized cells and gametes

• To become multicellular

As a cell grows, which increases faster: surface area or volume?

Volume increases faster than its surface area.

Why does a decreasing SA:V ratio force a cell to divide?

The cell membrane cannot keep up with the nutrient and ATP demands of the growing volume. It becomes saturated, so the cell must divide to survive.

What does a HIGH SA:V ratio mean, and why is it optimal?

• It means a large surface area relative to a small volume, allowing for efficient exchange across the membrane.

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What does a LOW SA:V ratio mean for cellular exchange?

It means a small surface area relative to a large volume. Membrane exchange cannot keep pace with cellular demand.

What is the primary function of cell cycle checkpoints?

• They send out enzymes to fix repairs in the genome/DNA so that mutations or damage are not duplicated.

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What are the three major checkpoints shown on the cell cycle diagram?

• G1/S checkpoint

• G2/M checkpoint

• Mitotic Spindle checkpoint

What is P53, and what is its nickname?

It is a transcription factor and protein nicknamed the "guardian of the genome."

How do cancer cells bypass the cell cycle regulation system?

They get rid of or mutate P53, allowing them to invade the cell cycle and replicate uncontrollably with nothing to stop them.

What is nondisjunction, and what condition can it lead to?

It is a failure in chromosome separation leading to aneuploidy (cells having more or fewer chromosomes than normal). Down Syndrome is a key example.

What do the abbreviations CDK and CAK stand for?

• CDK: Cyclin Dependent Kinase

• CAK: CDK activating kinase

What are Cyclins and CDKs, and how do they initially interact?

They are both proteins. Cyclin must bind to CDK to begin the activation process.

What specific role does CAK play in activating the Cyclin-CDK complex?

It acts as a phosphate-donating enzyme, taking a phosphate from ATP to phosphorylate the Cyclin-CDK complex.

Once the Cyclin-CDK complex is fully activated by phosphorylation, what does it do?

• It phosphorylates a target protein (tacks a phosphate onto it), which signals that the cell cycle is a "go."

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How does phosphorylation change the target protein to move the cell into the next cycle stage?

The added phosphate causes the target protein to change its conformation into an active form.

What happens to the cell cycle when there is no DNA damage?

The cell cycle proceeds normally and continues without interruption.

When DNA damage is detected, what pathway does p53 activate to halt the cell cycle?

• p53 activates p21, which acts as a roadblock to cause a temporary cell-cycle arrest.

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What happens to the cell cycle if the p53/p21 DNA repair is successful?

The roadblock is removed and the cell cycle continues.

What happens if the DNA damage persists and repair is unsuccessful?

p53 activates apoptosis (programmed cell death).

Where exactly does p53 bind when DNA damage is detected?

It binds to the promoter of the p21 gene to activate its transcription and translation.

What analogy do the notes use to describe p53's role in the cell?

• It acts as "the general" because it tells the cell what to do and where to go.

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What kind of molecule is p21, and what is its biochemical function?

It is a protein that acts as a cyclin-CDK inhibitor.

What specific target does the p21 protein look for after it is translated?

It searches for the active (phosphorylated) Cyclin-CDK complex and binds directly to it.

What is the consequence when p21 binds to the active Cyclin-CDK complex?

The complex becomes inactivated, meaning it can no longer phosphorylate the target proteins required for cell cycling.

What type of gene is TP53, and what does its protein (p53) do?

It is a tumour suppressor gene. Its protein prevents cells with damaged DNA from multiplying.

What is the primary benefit of functional p53 halting the cell cycle when DNA damage occurs?

It ensures that damaged DNA is not passed to daughter cells.

What two downstream outcomes can occur after functional p53 triggers a temporary cell cycle arrest via p21?

• Damage repaired: Cell cycle resumes.

• Damage persists: Apoptosis is triggered.

What is the immediate cellular consequence if p53 is lost or mutated following DNA damage?

There is no effective cell cycle arrest, meaning the damaged DNA is copied.

What is the step-by-step progression that occurs when a cell lacks functional p53?

Damaged DNA is copied → Mutations accumulate → Abnormal cells continue dividing → Cancer can develop.

What is ploidy?

• The number of sets of chromosomes that an organism has.

What does it mean for a human cell to be diploid (2n)?

It contains 2 sets of chromosomes (23 chromosomes per set; one set from mom and one from dad).

Are homologous chromosomes (e.g., Chromosome 1 from mom and Chromosome 1 from dad) completely identical?

• No. They are the same chromosome type, but they carry different genetic variations.

How many cell divisions occur in Mitosis vs. Meiosis?

• Mitosis: One division.

• Meiosis: Two divisions (Meiosis I and Meiosis II).

How do the daughter cells of Mitosis compare to Meiosis in number and ploidy?

• Mitosis: 2 diploid (2n) daughter cells.

• Meiosis: 4 haploid (n) daughter cells.

How does genetic similarity differ between Mitosis and Meiosis?

• Mitosis: Daughter cells are genetically identical to the parent and each other.

• Meiosis: Daughter cells are genetically different.

How many times does DNA replication (S phase) happen during Meiosis?

Only once, before Meiosis I begins.

What are the three primary sources of genetic variation in sexual reproduction?

1. Crossing over (Prophase I)

2. Independent assortment (Metaphase I)

3. Random fertilization

What is independent assortment?

The principle that chromosomes line up and separate during Metaphase I completely independently of how other chromosomes are doing so.

Why do DNA molecules wrap around histone proteins?

• DNA has an overall negative charge, while histones are positively charged, creating a natural electrostatic attraction.

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What is the golden rule for counting the number of chromosomes present?

Count the centromeres. If there is one centromere, it is considered one chromosome, regardless of whether it has one or two chromatids.

When a replicated chromosome (X-shape) splits during cell division, what happens to the chromosome count?

It goes from one chromosome (with two sister chromatids) to two separate chromosomes.

Why does crossing over only happen between non-sister chromatids?

Sister chromatids are identical copies. Crossing over between them would exchange identical DNA, resulting in no genetic variation.

Why is Meiosis I called a "reductional" division while Meiosis II is called an "equational" division?

• Meiosis I reduces ploidy from diploid to haploid (2n to n).

• Meiosis II maintains ploidy at the haploid level ( n to n)

What specifically separates during Anaphase I versus Anaphase II?

• Anaphase I: Homologous chromosomes separate.

• Anaphase II: Sister chromatids separate.

Do centromeres divide during Meiosis I?

• No. Centromeres do not divide in Meiosis I (sister chromatids stay together). They only divide during Meiosis II

In which of the two meiotic divisions do crossing over and independent assortment occur?

• Meiosis I only. Neither event occurs during Meiosis II.

Professor's Mutation Scenario: If an organism undergoes an extra replication cycle (S-phase twice: G1 to S to G2 to S to G2) before completing normal meiosis, what happens to the gametes?

• The DNA duplicates twice instead of once. This double replication would cause the resulting gametes to have double the normal amount of DNA (diploid gametes instead of haploid), leading to a polyploid zygote when fertilized.

Meiosis 1: How do chromosomes line up during Metaphase I?

They line up as homologous pairs side-by-side, allowing for independent assortment based on how they randomly align.

Meiosis 1: What specific structures pull apart during Anaphase I?

Homologous chromosomes pull apart to opposite poles (sister chromatids remain attached at their centromeres).

Meiosis 2:How do chromosomes line up during Metaphase II compared to Metaphase I?

In Metaphase II, chromosomes align individually single-file along the metaphase plate, rather than in pairs.

Meiosis 2: What specific structures pull apart during Anaphase II?

• Sister chromatids separate and are pulled to opposite sides of the cell

Meiosis 2: What is the final outcome at the end of Telophase II and Cytokinesis?

Four unique haploid (n) cells, each containing single-chromatid chromosomes.

What is a haplotype?

A region on the same chromosome where genes are close together and are often inherited together because they are linked.

How many chiasmata (crossover points) typically form between two homologous chromosomes?

Anywhere from 1 to 4 chiasmata during a single meiotic alignment.

How does physical distance between two genes on a chromosome affect the likelihood of crossing over?

• A crossover is much more likely to occur in larger spaces between genes (e.g., between Gene B and Gene C in the slide diagram) than in small spaces.

Based on the slide diagram, why are Gene A and Gene B highly likely to be inherited together?

They are located very close to each other on the chromosome (linked genes), leaving minimal space for a chiasma to form between them.

Exam Question Strategy: If parental genotypes are linked as AB and ab, which offspring genotypes are most likely to occur?

AB or ab (the linked combinations). Recombinant combinations like Ab or aB are highly unlikely because the genes are too close to separate easily.

During which phase of meiosis does independent assortment take place?

• Metaphase I, when homologous pairs align unpredictably along the cell equator.

How does independent assortment create different gamete combinations?

• The orientation of one homologous pair is completely independent of any other pair, leading to multiple possible combinations of maternal and paternal chromosomes (e.g., RY, ry, Ry, rY).

What biological process combines haploid DNA from two different parents into a single cell?

• Fertilization, which fuses a haploid oocyte (\(n\)) and a haploid sperm (n) to form a diploid zygote (2n)

What is aneuploidy?

• An abnormal number of chromosomes present in gametes after meiosis due to chromosomal mutations

What is non-disjunction?

The failure of chromosomes to separate properly during cell division (either homologous pairs in Meiosis I or sister chromatids in Meiosis II).

Exam Blueprint: How will different chromosomes be visually denoted on your exam questions?

By different physical sizes of the chromosome illustrations.

What is the gamete outcome if non-disjunction happens during the 1st meiotic division (Meiosis I)?

All 4 gametes are abnormal/defective (Two are (n+1) with an extra chromosome, and two are (n-1) missing a chromosome).

What is the gamete outcome if non-disjunction happens during the 2nd meiotic division (Meiosis II)?

• 2 normal gametes (n) and 2 abnormal gametes (One is (n+1) and one is (n-1).

During which stage of the cell cycle are chromosomes harvested to create a karyotype?

Metaphase, when chromosomes are at their most condensed state.

What are autosomes?

Chromosomes 1 through 22 (the non-sex chromosomes).

Why does the incidence of Down Syndrome (Trisomy 21) increase dramatically as a mother ages?

The meiotic checkpoint weakens as maternal age increases, making non-disjunction events more frequent.

The meiotic checkpoint weakens as maternal age increases, making non-disjunction events more frequent.

• Down Syndrome has the best survival outcome among autosomal aneuploidies. (Trisomy 13 and 18 usually cause severe defects where infants pass away within the first month or year of life).

What is Turner Syndrome?

A sex chromosome aneuploidy designated as 45,X, affecting females who have only one X chromosome.

What is Klinefelter Syndrome?

A sex chromosome aneuploidy designated as 47,XXY, affecting males born with an extra X chromosome.

What is Triple X Syndrome?

• A sex chromosome aneuploidy designated as 47,XXX, affecting females born with three X chromosomes.

What is Jacobs Syndrome / 47,XYY?

• A sex chromosome aneuploidy designated as 47,XYY, affecting males born with an extra Y chromosome.

Is a 45,Y chromosome configuration viable for life?

No, it is not viable. A cell cannot survive without at least one X chromosome because the X chromosome contains essential genes needed for survival.

Which three single nondisjunction errors can result in a 47,XXY child?

1. Maternal Meiosis I error (Homologous X chromosomes fail to separate)

2. Maternal Meiosis II error (Sister chromatids fail to separate)

3. Paternal Meiosis I error (X and Y chromosomes fail to separate)

Why cannot a single nondisjunction error in Paternal Meiosis II produce a 47,XXY child?

Paternal Meiosis II nondisjunction produces either XX or YY sperm. Fusing these with a normal X egg results in XXX or XYY, never XXY.

What type of egg combines with a normal Y sperm to produce an XXY child during maternal nondisjunction?

• An XX egg (caused by non-disjunction in either maternal Meiosis I or II).

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What type of sperm combines with a normal X egg to produce an XXY child during paternal nondisjunction?

An XY sperm (caused strictly by a paternal Meiosis I error).

By what specific process do animal gametes (sperm and egg) arise?

• Meiosis, which transitions the life cycle from the diploid phase (\(2n\)) to the haploid phase (\(n\)).

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By what process does a diploid zygote (\(2n\)) grow into a multicellular animal (2n)?

• Mitosis, which preserves the diploid chromosome count across cell divisions.

What event transitions the animal life cycle back from the haploid phase to the diploid phase?

• Fertilization, when a haploid sperm (n) and a haploid egg (n) fuse to form a diploid zygote (2n).

By what process do gametes arise in the plant and most fungi life cycle?

Mitosis (Unlike animals, whose gametes arise via meiosis).

By what process are spores formed in plants and most fungi?

• Meiosis, which transitions the cycle from the diploid sporophyte (2n) to a haploid spore (n)

What does a haploid spore develop into, and by what cellular process?

• It develops into a multicellular gametophyte (n) via mitosis.

What does a diploid zygote develop into in the plant life cycle?

A multicellular sporophyte (2n) via mitosis.

What is the major difference in how the zygote divides in "some fungi and algae" compared to plants and animals?

The zygote (2n) divides immediately by meiosis to produce haploid spores (n), meaning there is no multicellular diploid phase.

Do some fungi and algae have a multicellular sporophyte phase?

No. Their only multicellular structure is the haploid gametophyte (n).

By what process do gametes arise in this specific fungi and algae life cycle?

Mitosis (produced by the haploid gametophyte).

In which life cycles do gametes arise via mitosis vs. meiosis?

• Mitosis: Plants, all Fungi, and Algae.

• Meiosis: Animals only.

Which of the three life cycles lacks a multicellular diploid (2n) phase?

Some fungi and algae (the zygote undergoes meiosis immediately).

Which life cycle has distinct, multicellular phases for both diploid and haploid stages?

• Plants and most fungi (Alternation of generations between sporophyte and gametophyte).