Medical Biology Lecture 6 PDF

College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 6 Cell structures and function )The Nucleus) Learning Objectives: - Definition of nucleus and it functions. - The students will learn all information about the composition of nucleus. What is nucleus? The nucleus: is a mem...

College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 6 Cell structures and function )The Nucleus) Learning Objectives: - Definition of nucleus and it functions. - The students will learn all information about the composition of nucleus. What is nucleus? The nucleus: is a membrane-enclosed organelle found in eukaryotic cell that serves as the information processing and administrative center of the cell. The nucleus is the largest cellular organelle in animal cells. This organelle has two major functions: 1. It stores the cell's hereditary material (DNA) 2. Coordinates the cell's activities, which include growth, intermediary metabolism, protein synthesis, and reproduction (cell division). Usually the cells contain single nucleus , but few types of cells such as mammalian red blood cells, have no nuclei, and few others have more than one. The number of nucleus may vary from cell to cell. In mammalian cells, the average diameter of the nucleus is approximately 6 (μm), which occupies about 10% of the total cell volume. According number of nuclei present, cells are classified as:  mononucleate (e.g, smooth muscle cell).  Binucleate (e.g, liver cells, cartilage cells)  polynucleated (syncytial cells) (e.g, osteoclast). The shape of the nucleus normally related to the shape of the cell, but certain nuclei are almost regular in shape. The viscous liquid within it is called nucleoplasm, and is similar in composition to the cytosol found outside the nucleus. It appears as a dense, roughly spherical or regular organelle. The composition by dry weight of the nucleus is approximately: DNA 9%, RNA 1%, Histone Protein 11%, Residual Protein 14%, and Acidic Proteins 65%. 1 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 6 Structure of the nucleus The nucleus is composed of four main parts: 1. The nuclear envelope (Nucleolemma or Karyotheca). 2. Nuclear sap or nucleoplasm or Karyolymph 3. The chromatin network of fibers. 4. The nucleolus The nuclear envelope: The nucleus is surrounded by a nuclear envelope that separates the contents of the nucleus from the contents of the cytoplasm. It is composed of two layers of inner and outer membrane. Nuclear pores: in the envelope are small holes that control which ions and molecules (for example, proteins and RNA) can move in and out the nucleus. The outer membrane of the nucleus is continuous with the membranes of the rough endoplasmic reticulum (ER). 2 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 6 Nucleoplasm or Nuclear sap It is a fluid substance containing many particles and network .It is mainly composed of proteinaceous materials and is the main site for enzyme activity. It is separated from the cytoplasm by the nuclear envelope. Chromatin fiber DNA and proteins in the nucleus are called chromatin. They are present as a thread like structure in the nucleus. The Nucleolus The nucleolus is a densely stained structure found in the nucleus. It is not surrounded by membrane. It is a region inside the nucleus that is related to the production of ribosomal RNA molecules (rRNA) and the assembly of the ribosomes. Chromosome structure: Chromosomes are thread-like structures located inside the nucleus of animal and plant cells. Each chromosome is made of protein and a single molecule of deoxyribonucleic acid (DNA). The complex of DNA plus proteins is called chromatin. Protein associated with chromosome is classified into two broad groups: 1) Histone or basic protein Histones constitute about 80% of the total chromosomal protein. They are present in an almost 1:1 ratio with DNA. Histones play an important role in determining which genes are active. 2) Non histone proteins which make up to 20% of the total protein mass. Non histone protein includes many important enzymes like DNA and RNA polymerase. 3 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 6 Humans have 46 chromosomes, which occur in 23 pair .22 of these chromosome pairs are called autosome, which are found in both male and female. One pair of chromosomes is called the sex chromosomes, because this pair contains the genes that control gender .Male have the X and Y chromosome while female have both the X chromosome. In a typical body cell (somatic cell), are diploid (2n), meaning that most of their chromosomes come in matched sets known as homologues pairs. Homologues pairs: The two chromosomes in a homologous pair are very similar to one another and have the same size and shape. Most importantly, they carry the same type of genetic information: that is, they have the same genes in the same locations. However, they don't necessarily have the same versions of genes. That's because they may have been inherited from two different gene versions one from the maternal and one from the paternal side. The sex chromosomes The sex chromosomes, X and Y, determine a person's biological sex: XX specifies female and XY specifies male. These chromosomes are not true homologues and are an exception to the rule of the same genes in the same places. Aside from small regions of similarity needed during meiosis, or sex cell production, the X and Y chromosomes are different and carry different genes. In human the Y chromosome is much smaller than the X chromosome and carries only few genes of functional importance. One of the two X chromosomes in every cell in a female is randomly inactivated early in embryonic development (lyonization) to form what is called Barr body. Chromosome morphology: As a cell prepares to divide, chromosomes are seen to consist of two identical strands known as chromatids, or sister chromatids which are the result of DNA replication. 4 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 6 The two copies of a chromosome are called sister chromatids. The sister chromatids are identical to one another and are attached to each other. The attachment between sister's chromatids is at the centromere. Centromere is a localized region of the chromosome with which spindle fibers attached. It is known also as primary constriction or kinetochore. Each centromere divides the chromosome into short, p-arm and long, q arm. Types of chromosome according to centromere position: According to the relative position of centromere chromosomes are divided into four types. 1. Acrocentric. 2. Sub-metacentric. 3. Metacentric. 4. Telocentric. 5 College of Medicine Lecturer: Zahraa Ch. Hameed Medical Biology/ Lecture: 6 Telomeres Telomeres are sections of DNA found at the ends of each chromosome. They consist of the same sequence of bases repeated thousand times (TTAGGG).They protect the ends of chromosomes by forming a cap. They help to organize each of our 46 chromosomes in the nucleus and allow proper replication of the chromosomes. Method of Chromosomal study The different human chromosomes can be distinguished from one another by variety of techniques. Chromosomes are numbered from 1-22 according to their size ,1 being the largest and 22 being the smallest. Karyotyping: is the process by which photographs of chromosomes are taken in order to determine the chromosome complement of an individual, including the number of chromosomes and any abnormalities. Karyotype sometimes refer to the Complete set of chromosomes in an individual organism References: Madder, S., S., and Windelelspecht, M., Human biology, 15th ed. Mc Graw – hill, USA, 2018. Sylvia S. Mader, Biology, 6th ed. Mc Graw-Hill Education, USA,1999. 6