Module 3: Meiosis & Cancer Cells

What is the difference between asexual and sexual reproduction?

Asexual reproduction:

• offsprings are genetically identical

• fast and efficient

  • as long as environmental conditions do not change

Sexual reproduction:

• offsprings are genetically different from parents

• advantageous in changing environments

Diploid vs Haploid

Diploid (2n)

• cells have two sets of chromosomes, one from each parent

  • these are called homologous chromosomes

Haploid (n)

• cells have one set of chromosomes

Gametes

• haploid cells from each parent → sex cells

• produced via gametogenesis

• each gamete has 23 chromosomes (n = 23) → only one set

• humans have 23 pairs of chromosomes

  • that is 46 chromosomes in total (2n = 46)

How are haploid gametes produced?

• haploid gametes are produced via meiosis

1. Before DNA replication: Diploid

   • 2 homologous chromosomes

   • each chromosome has one chromatid

2. After DNA replication: Diploid

   • 2 homologous chromosomes

   • each chromosome has two sister chromatids

3. After Meiosis I: Haploid

   • 1 chromosome

   • each chromosome has two sister chromatids

4. After Meiosis II: Haploid

   • 1 chromosome

   • each chromosome has one chromatid

Life cycle of sexual organisms

1. the diploid phase (2n) begins at fertilization

   • this is when sperm and egg fuse

   • extends to meiosis

2. the haploid phase (n) begins with meiosis

   • ends at fertilization, when two gametes combine

Meiosis

• key process that turns one diploid cell (2n) into four haploid gametes (n)

• involves DNA replication and two rounds of cell division

2 types of meiosis

• Meiosis I

  • called the reduction division

  • chromosome number is cut in half → from diploid to haploid

  • homologous chromosomes separate

• Meiosis II

  • sister chromatids separate

  • similar to mitosis

  • ends with 4 haploid cells, each with one set of chromosomes

Meiosis I

• first division of meiosis

• separates homologous chromosomes

• this division reduces the chromosome number → from diploid (2n) to haploid (n)

• homologous recombination (crossing over) happens:

  • matching chromosomes exchange DNA segments

Prophase I

• long and complex phase

It can be divided into five stages:

1. Leptotene

   • chromatin starts to condense

   • long, thin chromosomes begin to become visible as threads

2. Zygotene

   • chromosomes further condense

   • homologous chromosomes pair up side by side → synapsis

   • they form bivalent or tetrad structures

3. Pachytene

   • chromosomes are very condensed

   • crossing over happens: where homologous chromosomes exchange DNA segments

4. Diplotene

   • homologous chromosomes begin to separate

   • but remain attached at crossover points → chiasmata

5. Diakinesis

   • chromosomes become fully condensed

   • nucleoli disappears

   • nuclear envelope breaks down

   • spindle starts to form

Crossing over during prophase I

• crossing over → exchange of DNA between homologous chromosomes

• occurs during prophase I

• cohesin proteins hold sister chromatids together during early prophase I

• synaptonemal complex stabilizes chromosomes during synapsis

  • when chromosomes pair up side by side

• small breaks are repaired by joining DNA segments of nonsister chromatid

• as the synaptonemal complex disappears later in prophase I

  • the crossover sites become visible as chiasmata

  • these sites hold homologs together

Metaphase I

• bivalents line up at the middle of the cell (spindle equator)

  • they attach via their kinetochores to spindle microtubules

  • they are randomly oriented

• homologous chromosomes stay connected at the chiasmata

  • site where crossing over happened

• having paired homologues at the spindle equator is specific to meiosis

  • in mitosis, individual chromosomes (not pairs) line up

Anaphase I

• homologous chromosomes separate and move to opposite spindle poles

• this separation is a key step that makes meiosis different from mitosis

• shugoshin → protein that protects cohesins at the centromere

  • ensure sister chromatids stay together until meiosis II

Telophase I

• haploid set of chromosomes arrive at each spindle pole

  • only one chromosome from each pair

• in some cells, nuclear envelope reforms around chromosomes

• chromosomes stay condensed until meiosis II begins

Cytokinesis

• two haploid cells are produced from dividing

• each cell has half the original number of chromosomes

  • but each chromosome still has two sister chromatids

Meiosis II

• second division of meiosis, resembles mitosis

• starts with two haploid cells

  • each cell contains one set of replicated chromosomes

  • each chromosome has two sister chromatids

• separates sister chromatids into 4 haploid cells

• often called “the separation division”

4 steps of meiosis II

1. Prophase II

   • short stage

   • resembles prophase of mitosis

   • spindles form and chromosomes stay condensed

2. Metaphase II

   • chromosomes line up at the middle of the cell (spindle equator)

   • like mitosis, but with half the number of chromosomes

3. Anaphase II

   • sister chromatids are separate and move to opposite poles

4. Telophase II & Cytokinesis

   • chromosomes arrive at the poles

   • nuclear envelope reforms around them

   • cell divides → forming 4 haploid cells (2 per original cell)

How does meiosis occur in human gamete development?

1. Sperm Cell Development

   • meiosis starts with diploid spermatocyte (2n)

   • it goes through meiosis I and II

     • produces 4 haploid spermatids (n)

   • these spermatids mature into sperm cells by:

     • losing most their cytoplasm

     • developing flagella

   • sperm are generated continuously through a male’s lifetime, as stem cells divide by mitosis

2. Egg Cell Development

   • meiosis starts with diploid oocyte (2n)

   • it goes through meiosis I and II

     • but only 1 out of the 4 cells become mature egg

     • the other 3 become small polar bodies

     • this occurs due to uneven splitting of cytoplasm through two rounds of meiosis

   • in amphibians, oocytes remain arrested in prophase I until prompted by a stimulus

What happens when meiosis fails?

• Aneuploidy

  • abnormal number of chromosomes

• Trisomy (2n + 1)

  • a gamete has an extra chromosome

  • after fertilization, the zygote ends up with 3 copies of one chromosome, instead of two

  • example: down syndrome (trisomy 21)

• Monosomy (2n - 1)

  • a gamete is missing one chromosome

  • the zygote ends up with only 1 copy of that chromosome

Why do we need meiosis?

1. Maintain the right number of chromosomes

   • meiosis creates haploid sex cells, each with half number of chromosomes

   • when fertilization occurs, the two haploid cells combine

   • without meiosis, the chromosome number would double

2. Increase genetic diversity

   • In prophase I, crossing over allows matching chromosomes to exchange DNA segments

   • this variation in offsprings → better adaptation in changing environments

Meiosis vs Mitosis

Mitosis

• growth, repair and replace cells

• Prophase

  • each condensing chromosome has 2 chromatids

  • they act independently

• Metaphase

  • individual chromosomes align at the metaphase plate

• Anaphase

  • chromatids separate

• Result

  • 2 cells, each with same number of chromosomes as original cell

  • daughter cells are identical

Meiosis

• 2 divisions → produces gametes

• Prophase

  • homologous chromosomes synapse, forming bivalent

  • crossing over occurs between non-sister chromatids

  • this produces chiasmata

• Metaphase

  • bivalents align at the metaphase plate

• Anaphase

  • chromosomes (not chromatids) separate

  • only in meiosis II, sister chromatids separate

• Result

  • 4 haploid cells, each with half as many chromosomes as original cell

  • daughter cells are genetically unique

What causes cancer?

• usually caused by mutations (changes) in DNA

  • this makes cells grow and divide uncontrollably

  • this includes environmental agents and lifestyle factors

Cancer-causing agents:

1. Carcinogens

   • chemicals, smoking

2. Radiation

   • X-rays, UV

3. Infectious agents

   • some viruses and bacteria

Carcinogens

• hundreds of known and suspected carcinogens

• example: polycyclic aromatic hydrocarbons (found in cigarette smoke)

• these chemicals can bind to DNA at important sites and cause mutations

• mutated DNA can lead to uncontrollable cell growth → cancer

Radiation

• Ionizing Radiation

  • can remove electrons from molecules

  • damages DNA molecules → cancer

  • example: X-rays

• UV Radiation

  • mainly absorbed by the skin

  • triggers pyrimidine dimer formation

    • covalent bonds form between adjacent pyrimidine bases in DNA

Viruses & Other Infections

• Oncogenic viruses → causes cancers

  1. Epstein-Barr virus (EBV)

     • associated w/ Burkitt lymphoma

  2. Hepatitis B & Hepatitis C

     • triggers some liver cancers

  3. Human papillomavirus (HPV)

     • associated w/ cervical cancer

• Other Infectious agents:

  1. Helicobacter pylori (H.pylori)

     • causes stomach cancer

  2. Flatworm infections

     • linked to bladder and bile duct cancers

Oncogenes

• it’s presence can trigger the development of cancer

• they are mutated forms of normal genes → proto-oncogenes

  • proto-oncogenes regulate cells growth and survivial

  • when mutated, they become oncogenes → cause excess cell division or block apoptosis (cell death)

• RAS → first oncogene identified

How do oncogenes form?

• from mutations in proto-oncogenes

• sometimes introduced by cancer-causing viruses

• multiple mutations of oncogene is needed to cause cancer

  • a single oncogene is not sufficient

5 mechanisms of converting proto-oncogenes into oncogenes

1. Point mutation

   • small change in DNA → abnormal (hyperactive) protein

2. Gene amplification

   • excess copying of gene → produces excess normal protein

3. Chromosomal translocation

   • 2 chromosomes exchange segments → excess normal protein or abnormal protein

4. Local DNA rearrangement

   • insertions, deletions, inversions → abnormal protein

5. Insertional mutagenesis

   • viral DNA inserted near proto-oncogene → produces excess normal protein

Tumour Supressor Genes

• genes that slow down cell division, repair DNA or trigger apoptosis

• their job is to prevent cancer by controlling cell growth

• cancer develops when tumour suppressor genes are lost or inactivated

• without them, cells grow out of control

2 types of tumour suppressor genes

• Gatekeeper genes

  • directly regulates cell division by inhibiting cell growth or induce apoptosis

  • example: RB, p53, APC

• Caretaker genes

  • repair DNA, sort chromosomes and maintain genetic stability

  • example: BRCA1, BRCA2

6 proteins that oncogenes produce

1. Growth factors

2. Receptors

3. Plasma membrane GTP-binding proteins

4. Non-receptor protein kinases

5. Transcription factors

6. Cell cycle / apoptosis regulators

4 examples of the tumour suppressor gene

1. Retinoblastoma (RB gene)

   • found on chromosome 13

   • produces RB protein, blocking cell cycle progression at G1 if there’s no growth signal

   • if both copies of RB gene are mutated → uncontrolled cell division → eye cancer

2. HPV

   • HPV produces E7 protein

     • this binds to and inactivates RB

   • loss of RB function → triggers cervical cancer

3. p53

   • most commonly mutated gene

   • normally detects DNA damage and either:

     • stops cell cycle

     • or triggers apoptosis

   • when p53 is mutated → damaged cells keep dividing

   • HPV has an oncogene that produces E6 protein

     • this targets p53 protein for destruction

4. APC

   • encodes for a protein in the Wnt signalling pathway

   • if both copies of APC are lost → Wnt stays permanently active

     • this prevents destruction of beta-catenin protein

     • stimulates continuous cell division

   • associated w/ familial adenomatous polyposis

     • condition where thousands of polyps develop in the colon

Carcinogenesis

• multi-step process that converts normal cells into cancer cells

6 Hallmarks of Cancer

• cancer cells develop a group of 6 traits

  • these traits are common to all forms of cancer

  • but each trait can be acquired differently

1. Self-sufficiency in growth signals

2. insensitivity to antigrowth signals

3. Evasion of apoptosis

4. Limitless replicative potential

5. Sustained angiogenesis

6. Tissue invasion & metastasis

Cancer

• disease caused by abnormal and uncontrolled cell growth

• cell division becomes separated from normal cell functions

  • this includes cell differentiation and death

3 types of cancers

1. Carcinomas

   • make up 90% of all cancers

   • arise from epithelial cells

2. Sarcomas

   • develop from supporting tissues, bones, cartilages, fat and muscle

3. Lymphomas and Leukemias

   • arise from blood and lymphatic system cells

   • lymphomas → tumours that grow as solid mass

   • leukemias → grow mostly in the bloodstream

Tumour

• resulting mass of tissues

2 types of tumour

1. Benign tumours

   • grow in one place, do not spread

   • rarely dangerous

2. Malignant tumours

   • can invade nearby tissues and spread to other body parts

How does cancer arise?

1. Anchorage-Independent Growth

   • normal cells only need to be attached to a surface to grow

   • cancer cells can grow on attached surfaces and when suspended in liquid or semisolid medium

2. Insensitive to Density-Dependent Inhibition

   • normal cells stop dividing when they touch neighbouring cells

     • they form a single layer

   • cancer cells continue to divide and pile up on eachother

3. Immortalization (no limits to divisions)

   • normal cells stop dividing after a set number of cycles due to telomere shortening

   • most cancer cells produce telomere → enzyme that adds telomere sequences to ends of DNA molecules

   • example: HeLa cells have been dividing since 1951

Multistep process of cancer development

1. Initiation

   • normal cells are converted into precancerous state

   • example: mutation from carcinogen

   • becomes sensitized to further change

2. Promotion

   • damaged cell is repeatedly exposed to cancer-promoting agents

   • longest and gradual process

3. Tumour Progression

   • the cell becomes fully cancerous, forming a tumour

   • the tumour grows and differentiates

How does cancer spread?

• Invasion

  • cancer cells move and penetrate into neighbouring tissues

• Metastasis

  • cancer cells enter the bloodstream and travel to distant sites

    • metastases → tumours formed in new locations

  • metastasis begins w/ angiogenesis → formation of new blood vessels around the tumour

Why do cancer cells metastasize?

1. Loss of cell adhesion

   • normal cells stick together using proteins like E-cadherin

   • in cancer, these adhesion proteins are missing or defective

2. Increased cell motility

   • cancer cells respond to signalling molecules (chemoattractants) and Rho GTPases

3. Production of proteases

   • these enzymes degrade protein-containing structures like basal lamina

     • this allows cancer cells to push through tissues and enter blood vessels

Genetic Instability in Cancer

• inactivation of tumour suppressor genes → genetic instability

  • cancer cells accumulate more mutations at a higher rate than normal cells

4 causes of genetic instability in cancer

1. Disruption in DNA repair

   • example: BRCA1, BRCA2

     • these genes help repair DNA breaks

2. Loss of p53 function

   • this defect removes a protective mechanism that prevents cells w/ damaged DNA from proliferating

   • without p53, damaged cells keep dividing → DNA errors

3. Chromosome sorting errors

   • if system is faulty → broken chromosomes

4. Defective spindle attachments

   • defects in proteins involved in attaching chromosomes to the spindle → missing or broken chromosomes