MPharm PHA115: Protein Synthesis & Cell Division

MPharm Programme: Protein Synthesis and Cell Division

• Course name: Protein Synthesis and Cell Division
• Course code: PHA115
• Lecturer: Dr Praveen Bhugra

Learning Objectives

• Students should be able to describe the sequences of events in protein synthesis, including gene expression, transcription, and translation.
• Students should be able to understand and explain the stages and significance of somatic and reproductive cell division.
• Students should understand and describe the signals that induce somatic cell division.

Introduction to Protein Synthesis

• Cells synthesize many chemicals to maintain homeostasis.
• Cellular machinery is devoted to producing large numbers of diverse proteins.
• Proteins determine the physical and chemical characteristics of cells.
• Proteins assemble cellular structures (plasma membrane, cytoskeleton, organelles), act as hormones, antibodies, and contractile elements in muscles.
• Other proteins are enzymes, that regulate chemical reactions in cells, and transporters that carry materials in the blood.

Introduction to Protein Synthesis (continued)

• DNA serves as a template for protein synthesis.
• Transcription: Information from DNA is copied into RNA.
• Translation: RNA information is used to synthesize a protein.

Transcription

• Three types of RNA are made from DNA: mRNA, rRNA, and tRNA.
• mRNA directs protein synthesis.
• rRNA joins with ribosomal proteins to form ribosomes.
• tRNA binds to amino acids and holds them in place on a ribosome until incorporated into a protein during translation.

Types of RNA Produced in Cells

• mRNA: codes for proteins
• rRNA: forms the core of ribosomes and catalyzes protein synthesis
• miRNA: regulates gene expression
• tRNA: serves as adaptors between mRNA and amino acids during protein synthesis
• Other small RNAs: participate in RNA splicing, telomere maintenance, and other processes

Transcription (continued)

• Enzyme RNA polymerase catalyzes DNA transcription.
• Only one DNA strand is used as a template for RNA synthesis.
• Transcription begins at a promoter (a specific nucleotide sequence).
• Transcription proceeds in a complementary manner.
• Transcription ends at a terminator (a special sequence).

Translation

• The nucleotide sequence in mRNA specifies the amino acid sequence of a protein.
• Ribosomes perform translation in the cytoplasm.
• A ribosome has a small subunit and a large subunit with binding sites for tRNA (P site and A site).
• The first tRNA molecule with a specific amino acid attaches to mRNA at the P site and the A site.
• The A site accepts next tRNA.
• Amino acids are linked together forming a peptide bond.
• The process continues until a stop codon is reached.

Cell Division

• Human cells undergo cell division.
• Somatic cell division: all body cells other than germ cells
• Reproductive cell division: produces gametes (cells for sexual reproduction)
• Somatic cell division replaces dead/injured cells and adds new cells during tissue growth.
• Somatic cell division involves mitosis and cytokinesis. This involves nuclear and cytoplasmic division respectively
• Reproductive cell division involves meiosis, a special two-step division reducing the number of chromosomes in the nucleus by half.

Somatic Cell Division

• The cell cycle is an orderly sequence of events.
• Human cells contain 23 pairs of chromosomes (46 total).
• Chromosomes that make up a pair are homologous chromosomes, which contain similar genes arranged in similar orders.
• Exception to this: the sex chromosomes (X and Y).
• Somatic cells contain two sets of chromosomes (diploid).
• The cell cycle has two major periods: interphase (when a cell is not dividing) and the mitotic (M) phase (when it is dividing).

The Cell Cycle

• Interphase has three subphases (G1, S, and G2).
• G1 checkpoint: signals to check the integrity of DNA, growth factors, nutrition and size of cell before entering DNA replication.
• S phase: DNA replication occurs.
• G2 phase: cell growth continues and protein synthesis occurs in preparation for division.
• The mitotic phase consists of mitosis and cytokinesis.
• Mitosis has four phases: prophase, metaphase, anaphase, and telophase.

Interphase

• G1 phase: interval between the mitotic phase and S phase
• Cell is metabolically active; replicates most organelles and cytosolic components, but not DNA. DNA replication of centrosomes starts here.
• S phase: Interval between G1 and G2, DNA replication occurs resulting in two identical cells.
• G2 phase: Interval between S and mitosis, continued cell growth, synthesis of enzymes and other proteins. Replication of centrosomes is completed.
• Original DNA molecule splits into two identical DNA molecules.

Mitotic Phase

• Mitosis and cytokinesis (includes Prophase, Metaphase, Anaphase, Telophase)

• Prophase: Chromosomes condense and become visible, spindle fibers (microtubules) start to form, the nuclear envelope disintegrates

• Metaphase: Chromosomes line up in the middle of the cell, attached to spindle fibers.

• Anaphase: Sister chromatids separate and move to opposite poles pulled by spindle fibers.

• Telophase: Chromosomes arrive at opposite poles, nuclear envelopes begin to reform, chromosomes decondense, the mitotic spindle breaks down.

Checkpoint Control System

• Cell cycle is controlled by STOP and GO chemical signals at critical points.
• Signals indicate if key cellular processes have been completed correctly.
• G1: checks DNA, growth factors, nutrition, and cell size.
• G2: checks if DNA replication is complete and DNA is intact.
• M: checks if chromosomes are attached to spindle fibers correctly.

Checkpoints (continued)

• G2/M checkpoint: checks DNA replication and integrity.
• G1/S checkpoint: regulates the cell cycle by monitoring growth factor signaling and nutritional conditions.
• Spindle checkpoint: monitors proper chromosome alignment and attachment of chromosomes to the spindle apparatus.

Checkpoints Control System (continued)

• Signals that promote cell growth & division proteins are called "promoting factors," external signals (like growth factors) are external.
• Primary mechanisms of control are phosphorylation (adding phosphate) through Kinase enzymes, and dephosphorylation (removing phosphate). Protein phosphorylation and dephosphorylation by kinases and phosphatases are major regulatory steps that control the cell cycle

Cyclin and Cyclin-Dependent Kinases (CDKs)

• CDKs and cyclins drive the cell cycle from one phase to the next.
• Proper regulation of the cell cycle is crucial for life.
• Genes for regulatory proteins are highly conserved through evolution.
• The related proteins are similar in yeast, insects, plants, and animals

Meiosis

• Meiosis is a specialized type of cell division that occurs in the gonads (ovaries and testes) to form gametes.
• Meiosis reduces the chromosome number by half.
• Gametes (sperm and egg cells) are haploid (n).
• Fertilization restores the diploid (2n) number of chromosomes.

Comparison of Mitosis and Meiosis

• Mitosis results in genetically identical cells, while meiosis results in genetically unique gametes.
• Mitosis occurs throughout life to aid growth and repair, while meiosis occurs at sexual maturity.
• Meiosis has two divisions (I and II), mitosis has one.

This quiz tests your understanding of protein synthesis and cell division. You will describe the processes of gene expression, transcription, translation, and the stages of cell division. Assess your knowledge of how these processes impact cellular functions.