Protein Synthesis and Cell Division PDF

**Protein Synthesis and Cell Division** **Learning Objectives** - Understand and explain in detail the sequences of events in protein synthesis with knowledge of gene expression, transcription and translation - Understand and explain the different stages, events and significance o...

**Protein Synthesis and Cell Division** **Learning Objectives** - Understand and explain in detail the sequences of events in protein synthesis with knowledge of gene expression, transcription and translation - Understand and explain the different stages, events and significance of somatic and reproductive cell division - Understand, explain and describe in detail the signal that induce somatic cell division **Introduction to Protein Synthesis** - Cell synthesize many chemicals to maintain homeostasis - Cellular machinery is devoted to synthesizing large numbers of diverse proteins - Proteins determine the physical and chemical characteristics of cells - Proteins - Help assemble cellular structures such as the plasma membrane, the cytoskeleton, and other organelles - Others serve as hormones, antibodies, and contractile elements in muscular tissue - Still others act as enzymes, regulating the rates of the numerous chemical reactions that occur in cells, or transporters, carrying various materials in the blood - DNA is used as a template for synthesis of a specific protein - **Transcription** - Information encoded in a specific region of DNA is transcribed (copied) to produce a specific molecule of RNA (ribonucleic acid) - **Translation** - The RNA attaches to a ribosome, where the information contained in RNA is translated into a corresponding sequence of amino acids to form a new protein molecule Diagram of a cell with text Description automatically generated **Transcription -- Protein Synthesis** - During the transcription, three types of RNA are made from the DNA template: 1. **Messenger RNA (mRNA)** directs the synthesis of a protein 2. **Ribosomal RNA (rRNA)** joins with ribosomal proteins to make ribosomes 3. **Transfer RNA (tRNA)** binds to an amino acid and holds it in place on a ribosome until it is incorporated into a protein during translation  **Transcription -- Protein Synthesis** - Enzyme RNA polymerase catalyses transcription of DNA - Only one of two DNA strands serves as a template for RNA synthesis - Segment of DNA where transcription begins, a special nucleotide sequence called a promoter - Transcription, bases pair in a complementary manner - Transcription of the DNA strand ends at another special end sequence called the terminator **Translation -- Protein Synthesis** - The nucleotide sequence in an mRNA molecule specifies the amino acid sequence of a protein - Ribosomes in the cytoplasm carry out translation - Small sub-unit of a ribosome has a binding site for mRNA - Large sub-unit has two binding sites for tRNA molecules, a P site and an A site the next  A diagram of a cell membrane Description automatically generated **Introduction to Cell Division** - Human body undergo cell division - Two types of cell division **somatic cell division** and reproductive cell - **Somatic cell**: any cell of the body other than a germ cell (reproductive cell) - In somatic cell division, a cell undergoes a nuclear division called **mitosis** and a cytoplasmic division called **cytokinesis** - Somatic cell division replaces dead or injured cells and adds new ones during tissue growth - Reproductive cell division is the mechanism that produces gametes - Cells needed to form the next generation of sexually reproducing organisms - Process consists of a special two-step division called meiosis, in which the number of chromosomes in the nucleus is reduced by half **Somatic Cell Division -- Cell Division** - **Cell cycle** is an orderly sequence of events in which a somatic cell duplicates its contents and divides in two - Human cells, such as those in the brain, stomach, and kidneys, contain 23 pairs of chromosomes, for a total of 46 - One member of each pair is inherited from each parent - The two chromosomes that make up each pair are called **homologous** **chromosomes** they contain similar genes arranged in the same (or almost the same) order - Exception to this rule is one pair of chromosomes called the sex chromosomes (designated X and Y) - In females the homologous pair of sex chromosomes consists of two large X chromosomes - In males the pair consists of an X and a much smaller Y chromosome - Somatic cells contain two sets of chromosomes, they are called diploid cells, symbolized 2n - Cell cycle consists of two major periods: **interphase**, when a cell is not dividing, and the **mitotic (M) phase**, when a cell is dividing - **The Cell Cycle**  **Interphase -- Cell Division** - The G1 phase is the interval between the mitotic phase and the S phase - The cell is metabolically active; it replicates most of its organelles and cytosolic components but not its DNA - Replication of centrosomes also begins in the G1 phase - For a cell with a total cell cycle time of 24 hours, G1 lasts 8-10 hours - Cells that remain in G1 for a very long time, perhaps destined never to divide again, are said to be in the G0 phase - The S phase, the interval between G1 and G2, lasts about 8 hours - During S phase, DNA replication occurs, resulting into two identical cells formed during cell division later in the cell cycle will have the same genetic material - The G2 phase if the interval between the S phase and the mitotic phase, lasts 4 to 6 hours - During G2, cell growth continues, enzymes and other proteins are synthesized in preparation for division, and replication of centrosomes is completed - When DNA replicates during the S phase, its helical structure partially uncoils, and the two strands separate at the points where hydrogen bonds connect base pairs - Original DNA molecule become two identical DNA molecules  **3 Checkpoint Control System -- G1, G2, M (Mitotic) Checkpoint** - Cell cycle controlled by STOP & GO chemical signals at critical points - Signals indicate if key cellular processes have been completed correctly Diagram of a control system Description automatically generated  **Checkpoints Control System -- Go-ahead signal** - Signals that promote cell growth & division - Proteins - Internal signals - "Promoting factors" - External signals - "Growth factors" - Primary mechanism of control - Phosphorylation - Kinase enzymes **Cyclin & Cyclin Dependent Kinases** - CDKs & cyclin drive cell from one phase to next in cell cycle - Proper regulation of cell cycle is so key to life that the genes for these regulatory proteins have been highly conserved through evolution - The genes are basically the same in yeast, insects, plants & animals (including humans)  A diagram of mitosis activity Description automatically generated **\ ** **Reproductive Cell Division** - Sexual reproduction, each new organism is the result of the union of two different gametes (fertilization), one produced by each parent - **Meiosis**, the reproductive cell division that occurs in the gonads (ovaries and testes), produces gametes in which the number of chromosomes is reduced by half - Gametes contain a single set of 23 chromosomes and thus are **haploid** (n) cells - Fertilization restores the diploid number of chromosomes - Meiosis occurs in two successive stages: **meiosis I** and **meiosis II**  **\ ** **Comparison between Mitosis and Meiosis** **REMEMBER this for MCQ !** **Mitosis** **Meiosis** ------------------------------ ------------------- --------------------- **Number of divisions** 1 2 **Number of daughter cells** 2 4 **Genetically identical?** YES NO **Chromosome Number** Same as parent Half of parent **Where** Somatic cells Germ cells **When** Throughout life At sexual maturity **Role** Growth and repair Sexual reproduction  SUMMARY -------------------------------------------------- ---------------------------------------------------------------------------------------------- --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Event Mitosis Meiosis DNA replication Occurs during interphase before nuclear division begins Occurs once, during the interphase before meiosis I begins Number of divisions One, including prophase, metaphase, anaphase, telophase Two, each including prophase, metaphase, anaphase, telophase Synapsis of homologous chromosomes Does not occur Synapsis is unique to meiosis; During prophase I, the homologous chromosomes join along their length, forming tetrads (groups of four chromatids); synapsis is associated with crossing over between nonsister chromatids Number of daughter cells and genetic composition Two, each diploid (2n) and genetically identical to the parent cell Four, each haploid (n), containing half as many chromosomes as the parent cell; genetically nonidentical to the parent cell and to each other Role in the animal body Enables multicellular adult to arise from zygote; produce cells for growth and tissue repair Produces gametes; reduces chromosome number by half and introduces genetic variability among the gametes