Bio D2 (Cell/Nuclear Division, Gene Expression, Water Potential)

D2.1 What is cell division?

Cell division is the process by which a parent cell divides into two daughter cells. It is essential for growth, repair, and reproduction.

D2.1 What is mitosis?

Mitosis is nuclear division that ensures two genetically identical daughter nuclei.

D2.1 What are the phases of mitosis?

1. Prophase: Chromosomes condense, nuclear envelope breaks down, centrosomes move to poles.

2. Metaphase: Chromosomes align at the equator, spindle fibers attach to centromeres.

3. Anaphase: Sister chromatids separate and move to opposite poles.

4. Telophase: Chromosomes decondense, nuclear envelope reforms, cytokinesis begins.

D2.1 Mnemonic for mitotic phases?

PMAT → Prophase, Metaphase, Anaphase, Telophase

D2.1 What happens to chromosomes during mitosis?

DNA supercoils, making chromosomes compact and visible under a microscope.

Example
A tangled string becomes shorter and thicker when twisted—just like chromosome condensation.

D2.1 How does cytokinesis differ in plants and animals?

• Animal cells: A contractile ring pinches the cell in two.

• Plant cells: A cell plate forms and develops into a new cell wall.

D2.1 Can cytokinesis be unequal?

Yes, examples include:

• Oogenesis: Produces one large egg and three small polar bodies.

• Yeast budding: A smaller daughter cell forms from the parent.

D2.1 What is the mitotic index?

The ratio of cells in mitosis to total cells, indicating tissue growth rate.

Example
If 25 out of 500 cells are in mitosis:
Mitotic index = 25/500 = 0.05 (5%)

It is useful as it:

• Assesses tissue growth

• Can indicate cancerous activity

D2.1 How do mitosis and meiosis differ?

• Mitosis: Maintains chromosome number, producing genetically identical cells.

• Mitosis: Diploid (2n) → Diploid (2n)

• Meiosis: Reduces chromosome number, creating genetic diversity.

• Meiosis: Diploid (2n) → Haploid (n)

D2.1 How do chromatids and chromosomes differ?

• Sister chromatids: Identical copies of a chromosome, joined at the centromere (before anaphase).

• Chromosomes: Individual structures after chromatids separate (from anaphase onward).

Common Mistake
Students often confuse sister chromatids with homologous chromosomes.

• Sister chromatids = Identical

• Homologous chromosomes = One from each parent, carrying different alleles

D2.1 How does meiosis create haploid cells?

• Meiosis I: Homologous chromosomes separate → Two haploid cells

• Meiosis II: Sister chromatids separate → Four haploid cells

D2.1 What happens in meiosis I?

• Prophase I: Chromosomes pair up (synapsis), crossing over occurs.

• Metaphase I: Homologous pairs align at the equator.

• Anaphase I: Homologous chromosomes separate (chromosome number halved).

• Telophase I & Cytokinesis: Two haploid cells form.

D2.1 What happens in meiosis II?

• Prophase II: Chromosomes condense again.

• Metaphase II: Chromosomes align at the equator.

• Anaphase II: Sister chromatids separate.

• Telophase II & Cytokinesis: Four haploid cells form.

D2.1 What is non-disjunction?

A failure of chromosomes to separate during meiosis, leading to extra or missing chromosomes.

Example
Down syndrome results from trisomy 21 (an extra chromosome 21).

D2.1 How does meiosis create variation?

1. Crossing over: Homologous chromosomes exchange genetic material.

2. Independent assortment: Random alignment of homologous chromosomes.

3. Random fertilization: Sperm and egg combinations create genetic diversity.

D2.1 What are the two types of tumors?

• Benign: Non-cancerous, slow-growing, do not spread.

• Malignant: Cancerous, grow rapidly, invade tissues, and metastasize.

D2.1 What causes tumors?

• Mutagens: Agents that cause DNA mutations.

• Oncogenes: Mutated genes that drive excessive division.

• Metastasis: The spread of cancer cells to other body parts.

D2.1 Why do cells proliferate?

• Growth: In embryos and developing tissues.

• Replacement: Skin cells are continuously replaced.

• Wound healing: Damaged tissues regenerate.

D2.1 What are the stages of interphase?

1. G1 phase: Cell growth and preparation for DNA replication.

2. S phase: DNA replication.

3. G2 phase: Further growth and preparation for mitosis.

Common Mistake
Interphase is NOT a resting phase—it’s when most cellular activity occurs!

D2.1 What are cyclins?

Proteins that regulate the cell cycle by activating cyclin-dependent kinases (CDKs).
They are important as they ensure the cycle progresses in the correct order and at the right time.

D2.1 How does cancer develop?

Mutations in genes that control cell division lead to uncontrolled growth.

Key factors in cancer:

• Proto-oncogenes → Oncogenes: Drive excessive division.

• Tumor suppressor genes (e.g., p53): Prevent division of damaged cells; mutations allow unchecked growth.

Example
p53 normally stops damaged cells from dividing. A mutated p53 leads to cancer.

D2.2 What is gene expression?

The process by which information encoded within genes is used to create proteins. It consists of two steps:

1. Transcription (DNA → mRNA)

2. Translation (mRNA → protein)

D2.2 What are the three key molecular components of gene expression?

1. Genome – The total genetic information in a cell, including all genes and non-coding DNA.

2. Transcriptome – All genetic instructions that have been transcribed into RNA (mRNA, tRNA, rRNA).

3. Proteome – The complete set of proteins expressed in a cell at a particular time.

D2.2 Do all cells in an organism have the same genome?

Yes, all cells in an organism have the same genome, but their transcriptome and proteome differ based on gene expression.

D2.2 What are the three levels at which gene expression can be controlled?

1. Translational control – Regulating the amount of protein produced.

2. Transcriptional control – Regulating the amount or activity of mRNA.

3. Genome-level control – Modifying DNA or histones to change gene accessibility.

D2.2 Why is gene regulation important?

It allows cells to specialize and differentiate, enabling different cell functions while maintaining the same genetic instructions.

D2.2 How is gene expression regulated at the mRNA level?

• mRNA degradation determines how long an mRNA transcript is available for translation.

• mRNA lifespan ranges from minutes to days.

• Longer-lasting transcripts produce more protein.

• mRNA is broken down by nucleases into nucleotides that the cell can reuse.

D2.2 How does mRNA degradation affect the transcriptome?

Since different mRNAs degrade at different rates, the transcriptome changes over time within a cell.

D2.2 What are transcription factors?

Proteins that regulate the binding of RNA polymerase to the promoter, controlling gene transcription.

Types are:

• Activator proteins – Bind to enhancer sites and increase transcription.

• Repressor proteins – Bind to silencer sites and decrease transcription.

D2.2 How do transcription factors influence cell differentiation?

• Different transcription factors are present in different tissues, leading to specialized cell types.

D2.2 How is transcription factor activity controlled?

Transcription factors respond to intracellular or extracellular signals:

Examples of extracellular signals affecting transcription are:

• Steroid hormones – Bind to receptors inside the cell and act as transcription factors.

• Peptide hormones – Bind to receptors on the plasma membrane and control transcription via second messengers.

Examples of intracellular signals affecting transcription are:

• Inducer molecules like lactose, which binds to a repressor protein and removes transcription suppression, allowing lactose metabolism genes to be expressed.

D2.2 What is epigenesis? How does epigenesis allow differentiation?

Epigenesis is the development of an organism from an undifferentiated zygote to a complex multicellular structure via differential gene expression.

It allows differentiation as:

• Different genes are activated in different cells.

• This alters cell characteristics (phenotype) without changing the DNA sequence (genotype).

D2.2 What controls epigenetic changes?

Chemical modifications called epigenetic tags that regulate gene activity.

D2.2 What is DNA methylation?

The addition of a methyl (-CH3) group to DNA (usually at CpG islands) or histones, which affects transcription.

D2.2 How does DNA methylation affect transcription?

• Methylation of DNA promoters prevents RNA polymerase from binding, reducing transcription.

• Methylation of histones maintains their positive charge, making DNA more tightly packed and less accessible for transcription.

D2.2 What happens when methylation is removed?

• DNA becomes loosely packed (euchromatin) and is more accessible for transcription.

• Different cell types have different methylation patterns, controlling gene activity.

D2.2 How can environmental factors influence DNA methylation?

Air pollutants, such as nitrogen oxides and polycyclic aromatic hydrocarbons (PAH), can alter methylation.

D2.2 How does air pollution affect gene expression?

• Decreases overall methylation but increases methylation at specific sites.

• Affects immunoregulatory genes, leading to increased inflammation and altered immune responses.

• Can contribute to high blood pressure, asthma, and cancer.

• They affect the enzyme DNA methyltransferase (DNMT), which adds methyl groups to DNA.

D2.2 What do twin studies reveal about epigenetics?

• Monozygotic (identical) twins start with identical genomes but develop different epigenetic profiles over time due to environmental influences.

• Comparing their DNA methylation patterns helps identify genes linked to diseases.

D2.2 What happens to epigenetic tags during fertilization?

• Most tags are erased to reset the embryo’s epigenome.

• This ensures the embryo can develop into all cell types.

D2.2 What are imprinted genes?

• A small number of genes retain their epigenetic tags during gamete formation.

• These genes influence offspring traits based on which parent they come from.

D2.2 How do imprinted genes explain liger and tigon size differences?

1. Liger (male lion × female tiger)

   • Male lions have imprinted genes promoting growth.

   • Female tigers do not have imprinted genes to restrict growth.

   • Result: Ligers grow very large.

2. Tigon (male tiger × female lion)

   • Male tigers lack growth-promoting imprinted genes.

   • Female lions have imprinted genes to restrict growth.

   • Result: Tigons are smaller than ligers.