Genetics Exam #2

Houke

• 1600

• Looked under a microscope (cork) and saw cells

Schleiden and Swan

• Schleiden(botanist)

• Swan (animals)

• 1840

• Cells come from other cells

Mitosis and Meiosis

• Late 1800’s

• Well described

Chromosomes

• 2 chromatids (sister chromatids) held together by the centromere

• Structure:

  • Size

  • Centromere location

• Bonding

  • Banding Pattern

  • 2* constrictions

• Karyotype: Number of chromosomes

Metacentric

In the middle

Sub-metacentric

Near the middle

Acrocentric

Near the end

Telocentric

Way to the end

Giemsa Stain

• G

• Most common

Reverse Giemsa

• R

C

Satellites

Little nobs at the end of a chromosome

Mitosis

• 1 Cell → 2 Cells

• 2N → 2N

• 2 chromatids/chromosome → 1 chromatid/chromosome

• Identical to one another, but also the parent cell

  • Identical in chromatids, alleles, and genes

• Doesn’t change the number of chromosomes

Mitosis: Prophase

• Chromosomes condense

• 2N = 4

Mitosis: Metaphase

• Line up in the middle

• Spindle fibers radiate from the pole and attach to the centromeres

Mitosis: Anaphase

Pulls sister chromatids apart

Mitosis: Telophase

• 2 big and 2 small on one side

• Nuclear membrane being formed

Mitosis: Cytokinesis

Cell divides and you end up with 2 cells

Interphase

• Do not see chromosomes

  • Are coiled

• We see chromatin (stain)

Interphase stages

• G₁

• S

• G₂

Meiosis

• Reduction Division

  • Reduces the number of chromosomes

  • 2N → 1N

• 1 Cell → 2 Cells

• 2 chromatids → 2 chromatids

Meiosis: Prophase I

• 2N = 4

• At the end → Synapse of Homologous Chromosomes

• Divide into stage:

  1. Leptotene

  2. Zygotene

  3. Pachytene

  4. Diplotene

  5. Diakinesis

Leptotene

• Condensation of chromosomes

• Hasn’t condensed enough to see individual chromatids

Zygotene

• Synapsis of homologous chromosomes

• Hasn’t condensed enough to see individual chromatids

Pachytene

• Condensed enough to see individual chromatids

• Synapsis of big and small chromosomes

• Bivalent: 2 homologous chromosomes with 2 homologous chromatids

• Tetrad: 4 chromatids

Diplotene

• Chiasmata: Arms of homologous chromosomes get entangled

• Chiasma leads to cross over (genetic recombination)

Diakineses

• Centromeres of the homologous chromosomes are being pulled apart, but the arms of the chromatids are holding them together

Meiosis: Metaphase I

• Synapsed homologous chromosomes align along the central plate.

• Spindle fibers attach to the centromeres of chromosomes

Meiosis: Anaphase I

• Pulls homologous chromosomes to opposite poles via spindle fibers which contract

• 2 Haploid cells

• Dyad

Meiosis: Telophase I

• Nuclei forming

• Each has 2 chromatids

• Haploid

Cytokinesis

2 cells

Meiosis II

• Mitotic Division

  • Does not change the number of chromosomes

  • Changes the number of chromatids per chromosome

• 2 Cells → 4 Cells

• 1N → 1N

• 2 chromatids → 1 chromatid

Meiosis: Prophase II

• 1N

• Chromosomes condense and are visible

• Spindle fibers begin to form

• Nucleolus disappears and nuclear membrane breaks down

Meiosis: Metaphase II

• Chromosomes align along the central plate

• Spindle fibers attach to the centromeres of chromosomes

Meiosis: Anaphase II

• Spindle fibers contract

• Sister chromatids are pulled apart

  • Sister chromatids are structurally and genetically identical

• Monad

Meiosis: Telophase II

• Each nucleus is haploid

• Each chromosome has 1 chromatid

Meiosis: Cytokinesis II

• 4 cells

• 1N

• 1 chromatid

Cell Cycle

• Mitosis → Interphase → (Back to) Mitosis

• G₁ → S → G₂ → M

G₁

• Growth and preparation of the chromosomes for replication

• Cyclin D-CDK4 and CDK6 → Affecting pocket proteins → Freeze E2F → Turn on gene

S

• Doubling of genome (DNA)

• Synthesis of DNA and duplication of the centromeres

• Cyclin E—CDK2

• Cyclin A—CDK2

  • Stop further DNA replication

G₂

• Preparation for Mitosis

• Cyclin B — CDK1

M (M-phase)

• Halving of the genome

• Mitosis

• CDK1

Cyclin-Dependent Kinases

• Add (or take away) phosphate groups (PO₄) to a variety of protein substrates that control processes in the cell cycle

• Always present but not doing its job

• To do a job(needs) → Cyclin CDK → Activate E2F

  • E: Eukaryotic

  • F: Transcription Factor

Pocket Proteins

• Bind to transcript proteins to turn them off

• Cyclin CDK target pocket proteins so they release E2F

1. CDK E → CDK4 and CDK6

2. Cyclin D → CDK2

3. SPF: S-phase promoting factor

4. S: Cyclin A → CDK2

5. G₂: Cyclin D → CDK1

Steps in the cycle

1. G₁ cyclins bind to their CDK’s and signal the cell to prepare the chromosomes for replication

2. SPF (A cyclins bound to CDK2) enters the nucleus and prepares the cell to duplicate DNA (and centromeres)

3. DNA replication continues → Cyclin E is destroyed → Mitotic cyclins begin to rise in G₂

4. Translocation of M-phase promoting factors (Cyclin B — CDK1) into nucleus initiates

   • Assembly of mitotic spindle

   • Breakdown of nuclear envelope

   • Cessation of gene transcription

   • Condensation of chromosomes

5. Metaphase of Mitosis

6. M-phase promoting factor activates APC/C

   • Starts anaphase

   • APC/C (binds to) → Ubiquitin (destroys) → Cyclin B—CDK1

     • APC/C binds Ubiquitin to Securin → No Securin → Separase(Separin) activates → Breakdown cohesion

Checkpoint: G₁

• Checks for DNA damage

• Restriction point

• ATM and ATR

  • Scan for errors

  • Shut down Cyclin E

  • Stop Cyclin D—CDK2 from binding

  • Activates p53 a tumor surpressor

Checkpoint: G₂

• Checks for DNA damage

• Checks for DNA replication errors

Checkpoint: M

• Detect failure in spindle fibers → Attach to kinetochores

• Detect improper alignment → Block cytokinesis → Trigger apoptosis

Checkpoint: G₀

• Pause point

• Doesn’t allow for replication under unproper conditions

Euploid

• Add sets of chromosomes

• 1N: monoploid

• 2N: diploid

• 3N: Triploid

• 4N: Tetraploid

• … polylpoid

• Autopolyploid

Aneuploid

• Add or remove number of individual chromosomes

Euploid: Humans

• Triploid

  • Fatal

  • Almost always arise from double fertilization(1%)

• Tetraploid

  • Fatal

  • Failure of Mitosis zygote

Euploid: Plants

• Raphano Brassica