last 4 lectures biochem

Meiosis

• Meiosis has two consecutive divisions: meiosis I and meiosis II, with no DNA synthesis between them.
• Genetic diversity in meiosis results from crossing-over during synapsis, independent assortment of chromosomes, and random fertilization.

Meiosis I

• In prophase I, chromosomes condense, spindles form, and the nuclear envelope breaks down.
• A difference to mitosis: homologous chromosomes join together at regions called chiasmata, and crossing over occurs, where regions of homologous chromosomes are swapped.
• In metaphase I, chromosomes align in the middle of the cell, with homologous chromosomes aligned together.
• In anaphase I, chromosomes move to poles, and homologous chromosomes separate, while sister chromatids remain attached at the centromere.
• In telophase I, two haploid cells form, each with paired sister chromatids.

Meiosis II

• Meiosis II is similar to mitosis, with the exception that four haploid cells are produced.

Differences between Mitosis and Meiosis

• Mitosis: one division, two diploid daughter cells, no crossing over, daughter cells genetically identical to parent cell.
• Meiosis: two consecutive divisions, four haploid daughter cells, crossing over present, genetic diversity results.

Cellular Biology and Biochemistry

Organelles and Cell Division

• Organelles: nucleus, endoplasmic reticulum, Golgi apparatus, mitochondria, Plastids are specialized organelles found in plant cells that are responsible for photosynthesis and other metabolic processes. They are present in green plants, algae, and some bacteria. According to their pigmentation and function, plastids can be categorized into several types, including:

  • Chloroplasts: responsible for photosynthesis and are found in green plants
  • Leucoplasts: involved in the synthesis of starch and other carbohydrates, found in etiolated plants
  • Chromoplasts: involved in the breakdown of chlorophyll and the synthesis of carotenoids, found in ripening fruits and flowers
  • Amyloplasts: involved in the synthesis of starch grains, found in potatoes and cereals
  • Elaeoplasts: involved in the synthesis of triglycerides, found in plants that store lipids

  Plastids have their own DNA, known as chloroplast DNA (cpDNA) or plastid DNA (ptDNA), which is inherited from the maternal parent. The genetic information encoded in cpDNA or ptDNA is responsible for the synthesis of proteins involved in photosynthesis and other metabolic processes.

  (including chloroplasts).

• Cell division: mitosis (cellular growth) and meiosis (reproduction).

Cell Junctions and Communication

• Cell junctions: connections and communication between cells.
• Connections: cells in multicellular organisms need to be stably attached to each other.
• Communication: cells need to communicate and coordinate, through signalling molecules or direct cell-to-cell contact.

Sex Chromosomes

• In mammals: females = XX, males = XY.
• In birds: females = ZW, males = ZZ.
• In platypuses: females = XXXXXXXXXX, males = XXXXX/YYYYYY.

Haploid and Diploid Cells

• Haploid (n): one set of chromosomes.
• Diploid (2n): two sets of chromosomes.
• Examples: females = diploid, males = haploid in hymenoptera (bees, ants, etc.).

Importance of Meiosis

• Meiosis produces haploid gametes (sperm and eggs) in animals and haploid spores in plants.
• These fuse to generate diploid offspring that grow through mitosis.

Signal Reception

• Receptors are specific for certain signals, ensuring only correct cells respond.
• A ligand is a specific signaling molecule that binds to a specific receptor.

Types of Receptors

• Cell-surface receptors:
  • Ligand-gated ion channel receptors
  • G protein-coupled receptors
  • Enzyme-coupled receptors
• Cytoplasmic receptors:
  • Steroid receptors

Ligand-Gated Ion Channel Receptors

• System involves: signal molecule (wide range possible), gated ion channel
• When the signaling molecule binds, the channel opens, allowing specific ions to flow.
• The gate must return to the closed position at the end of the signal.

G Protein-Coupled Receptors (GPCRs)

• Found associated with the plasma membrane in all eukaryotes.
• 800 different GPCRs in humans, with 60% of drugs targeting GPCR pathways.
• System involves: signal molecule (ligand), G protein-coupled receptor, G protein, enzyme.
• G proteins are GTP-binding proteins that switch from inactive to active state upon ligand binding.

G Protein-Coupled Receptors (GPCRs) - Activation

• The signal molecule binds to the receptor, causing a shape change that allows the G protein to bind.
• The G protein is activated by switching GDP for GTP.
• Activated G protein then activates an enzyme, triggering a cellular response.

G Protein-Coupled Receptors (GPCRs) - Deactivation

• The signaling system is turned off and reset when the G protein is deactivated by hydrolysis of GTP to GDP.

Examples of G Protein-Coupled Receptors

• Adrenaline receptor
• Rhodopsin: photoreceptor

Cellular Signalling

• Sometimes the cell that detects the signal is far from the responding cells.
• The signal is relayed (transduced) as an electrical signal through the spine.

Kinds of Cellular Signalling

• Direct cell-to-cell signalling:
  • Cell junctions
  • Cell-cell recognition
• Local signalling:
  • Paracrine signalling
  • Synaptic signalling
• Long-distance signalling (endocrine signalling), mediated by hormones.

Signalling Molecules

• Can be gaseous, water-soluble, lipid-soluble, or proteins.
• Examples: ethylene, estrogen, insulin.

Reception - Transduction - Response

• Signal reception involves a signaling molecule binding to a receptor, changing its shape.
• Signal transduction involves the transmission of the signal.
• Signal response involves a change in protein activity or expression.

G Protein-Coupled Receptors (GPCRs)

• G proteins are GTP-binding proteins that play a crucial role in signal transduction pathways.
• The system involves a signal molecule (ligand) binding to the G-protein-coupled receptor, causing a shape change that allows the G protein to bind.
• The G protein is activated by switching GDP for GTP.

Signal Transduction Pathways

• Signal transduction pathways can involve only proteins or proteins and second messengers.
• Examples of signal transduction pathways include phosphorylation cascades.
• The process involves signal reception, signal transduction, and signal response.

Types of Receptors

• Cell-surface receptors include ligand-gated ion channel receptors, G protein-coupled receptors, and enzyme-coupled receptors.
• Cytoplasmic receptors include steroid receptors.
• 30% of human proteins are cell surface receptors.

Ligand-Gated Ion Channel Receptors

• The system involves a signal molecule binding to the gated ion channel, opening it and allowing specific ions to flow.
• It is crucial that the gate returns to the closed position at the end of the signal.
• Examples include chemical synapses, where the ligand is a neurotransmitter (e.g., dopamine, acetylcholine) and the receptor is a transmitter-gated ion channel.

G Protein-Coupled Receptors (GPCRs)

• GPCRs are signal receptors found associated with the plasma membrane and are found in all eukaryotes.
• There are 800 different GPCRs in humans, and 60% of drugs target GPCR pathways.

Cellular Signalling

• For any given cell, there are hundreds of different signals it can respond to, hundreds of different receptors, and thousands of different pathways that can be activated, leading to lots of different responses.
• Signalling molecules can be gaseous, water-soluble, or lipid-soluble, and can be small molecules or proteins.
• Long-distance signalling is also known as endocrine signalling, which is mediated by hormones.

Signal Transduction

• The process involves signal reception, signal transduction, and signal response.
• A signalling molecule binds to a receptor, which changes shape, allowing the signal to be transduced and ultimately leading to a change in protein activity or expression.

Cell Signalling

• Signal transduction pathways involve three main themes: signal reception, signal transduction, and signal response.

Signal Reception

• Receptors are usually found in the plasma membrane.
• Receptors change shape when a ligand binds.
• Receptors can be switched off by ligand dissociation.

Signal Transduction

• Protein switches, such as GTP/GDP and phosphorylation/dephosphorylation, play a crucial role.
• Small molecules, known as second messengers, activate other proteins.

Signal Response

• Effectors are responsible for the signal response.
• Signal transduction pathways can have various outcomes, including:
  • Altered metabolism
  • Altered gene expression
  • Altered cell movement or shape

Amplification

• Amplification allows a small initial signal to be massively increased.
• Example: 1 molecule of epinephrine can lead to 100 million molecules of glucose-1-phosphate.

Common Theme

• Turning on and off the signal is a common theme in signal transduction pathways.
• Receptors, second messengers, and enzymes can all be turned on and off.

Response Outcomes

• Altered metabolism: involves changes in cellular metabolic processes.
• Altered gene expression: involves changes in gene transcription, often involving growth factors and steroid receptors.
• Altered cell movement or shape: involves changes in the cytoskeleton, and is involved in processes such as muscle contraction, wound healing, and embryo development.

Response Timing

• Extracellular signals can act slowly or rapidly, depending on the signal.
• Examples: adrenaline (rapid) and growth factors (slow).