The Structure of the Cell (Part IV) Lecture Notes PDF

The Structure of the Cell “Part IV” By Dr. Amany Abd El Fattah Mohamed Lecturer of Medical Histology & Cell Biology  Learning outcomes ❑ At the end of the lecture, the students will be able to: 1. Mention characters of nucleus and its components....

The Structure of the Cell “Part IV” By Dr. Amany Abd El Fattah Mohamed Lecturer of Medical Histology & Cell Biology  Learning outcomes ❑ At the end of the lecture, the students will be able to: 1. Mention characters of nucleus and its components. 2. Describe L.M & E.M of nuclear envelope. 3. Describe L.M & E.M of chromatin. 4. Describe L.M & E.M of nucleolus 5. Describe structure of nuclear sap 6. Define interphase and its stages. Q: What is the most important structure inside the cell? Why?  Learning outcome 1 Mention characters of nucleus and its components.  Nucleus It is the master organelle inside the cell. It has important functions: 1.Directs all the metabolic activities. 2.Plays an important role in heredity and cell division.  L.M.: Variable in number, size, shape and position in different cells, may be: ✓ Single or multiple ✓ Rounded, oval, flat or irregular. ✓ Central, eccentric, or peripheral. ▪ H&E: basophilic due to DNA content, may be: A.Open face nucleus: Pale-stained, its details are seen, present in active cells (e.g. liver, neurons) B.Condensed nucleus: deeply stained, its details are not seen, present in inactive cells (e.g. lymphocyte).  E.M.: Nucleus Nuclear Nuclear Chromatin Nucleolus sap envelope  Learning outcome 2 Describe L.M & E.M of nuclear envelope.  1- Nuclear envelope A membrane that encloses the nucleus and separates it from the surrounding cytoplasm. L.M.: single basophilic line surround the nucleus (has chromatin attached to its inner surface).  E.M.: Formed of 2 membranes, outer & inner nuclear membranes. Interrupted by holes called nuclear pores closed by diaphragm.  Functions: 1. Separates the nucleus from the surrounding cytoplasm. 2. Exchange of material between nucleus and cytoplasm through nuclear pores.  Learning outcome 3 Describe L.M, E.M & types of chromatin.  2- Chromatin Basophilic material of which chromosomes are formed. Formed of a nucleoprotein (DNA conjugated with histone protein)→chromosomal thread. L.M.: basophilic granules called chromatin granules. E.M.: 2 types: 1. Heterochromatin (condensed chromatin). 2. Euchromatin (extended chromatin).  Euchromatin Heterochromatin Structure Represents the extended parts Represents the coiled parts of of the chromosomal threads. the chromosomal threads. Activity Its genes are active. Its genes are inactive. E.M. It is seen, only with high Electron dense granules (20-30 magnification, as shades of nm in diameter). gray. Function Directs protein synthesis in the Does not direct protein cell. synthesis in the cell.  Learning outcome 4 Describe L.M & E.M of nucleolus  3- Nucleolus Nucleoli are one or more rounded basophilic bodies present in interphase nuclei. L.M.: Rounded basophilic mass inside the nucleus.  E.M: ▪ Irregular spongy mass, appears as twisted and folded cord, not limited by a membrane. ▪ Surrounded by aggregations of DNA heterochromatin known as nucleolus-associated chromatin. ▪ It contains segments of extended chromatin (DNA) carrying the genes for transcription of rRNA called Nucleolar Organizer Genes, present on 5 pairs of chromosomes (13, 14, 15, 21 ,22).  Function: 1. Synthesis of rRNA around nucleolar genes. 2. rRNA is conjugated with protein and stored in the granular part until it migrates through nuclear pores to reach the cytoplasm as ribosome subunits. *N.B.: Protein synthesis does not occur in the nucleus… Why? As the assembly of ribosome subunits into mature ribosomes takes place only in the cytoplasm.  Learning outcome 5 Describe structure of nuclear sap  4- Nuclear sap. Semifluid clear solution in which chromatin and nucleoli are suspended. Structure: It contains: 1. Proteins and nucleoproteins. 2. Enzymes catalyzing the synthesis of DNA and RNA. Function: It serves as the medium in which ribonucleic acids (rRNA, mRNA and tRNA) move towards nuclear pores.  Learning outcome 6 Define cell cycle, interphase and its stages.  Cell Cycle A cycle of stages that cells pass through to allow them to divide and produce new cells. It includes: 1. Interphase. 2. Cell division. ❖Somatic non-dividing cell contains 46 s- chromosome in the form of long thin threads. ❖S- chrom. is single stranded chromosome or chromatid. ❖D- chromosome is double S- chromosome stranded chromosome. D- chromosome  The period between two successive cell divisions. Cell prepares itself for division. Stages: G1 stage. S stage. G2 stage.  G1 S- stage G2 ❖ Growth of the S – chromosome Pair of s-chromosomes duplication. form one d-chromosome. newly formed 92 of S - Duplication of centriole. daughter cell. chromosome. Check for error in ❖ 46 of S - replication & its chromosome. correction. 46 of d –chromosome.  Summary and wrap up Nucleus is the master organelle in the cell. It is formed of nuclear envelope, chromatin, nucleolus & sap. Nuclear envelope is 2 membranes interrupted by pores. Chromatin is the genetic material, having 2 types, heterochromatin & euchromatin. Nucleolus is the site for formation od rRNA. Interphase is the period between 2 successive cell divisions.  References or further readings Junqueira's Basic Histology Text & Atlas (15th ed.). https://www.youtube.com/watch?v=fV-sG3od1k0 https://www.youtube.com/watch?v=nD2Fd0KyICA