1. The importance of teaching students how to complete an assignment?
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2. Of which structures is the nuclear membrane composed?
Eukaryotic cells have nucleus delimited by two juxtaposed membranes that continue with the membrane of the endoplasmic reticulum. The nuclear membrane, or karyotheca, presents pores through which substances pass. There are also ribosomes adhered to its external surface.
The nucleolus is a small and optically dense region in the interior of the cell nucleus. It is made of ribosomic RNA (rRNA) and proteins. One nucleus can have one or more nucleolus.
Cell Nucleus Review - Image Diversity: nucleolus
4. Do phylogenetically proximal species have cells with proximal chromosome counts?
The number of chromosomes typical of each species is proximal for phylogenetically proximal species (for example, orangutan, gorilla, chimpanzee and human). But it is not impossible that evolutionary distant species, like rat and oat, bears similar karyotypes and the same total number of chromosomes.
Even presenting equal number of chromosomes evolutionary distant species have radically different characteristics since the quantity and the sequence of nucleotides that compose their respective DNA molecules are quite different.
The human haploid cell is the gamete (egg cell and sperm cell). The human gamete has 22 autosomes and 1 allosome, i.e., 23 chromosomes. The diploid cell is the somatic cell and it has 44 autosomes and 2 allosomes, i.e., 46 chromosomes.
Gametes have one sex chromosome and somatic cells have two sex chromosomes.
6. What is the other name given to sex chromosomes? What is the function of sex chromosomes?
Sex chromosomes are also called allosomes (the other chromosomes that are not sex chromosomes are called autosomes).
Sex chromosomes get such name because they have genes that determine the sex (male or female) of an individual. Sex chromosomes also have genes related to other biological functions.
Except for clones (individuals created from nucleus transplantation, like the Dolly sheep) and monozygotic twins, it is very improbable the genomes of two individuals of the same species and generated by sexual reproduction to be identical. Nevertheless the karyotypes of two normal individuals of the same species and of the same sex are always identical. The human normal karyotype is represented by the formula 44+XX for women and 44+XY for men.
8. What is the difference between the concepts of karyotype and genome?
Genome is the set of DNA molecules that characterizes each living being or each species. The concept then includes the specific nucleotide sequence of the DNA molecules of each individual or species. Karyotype is the set of chromosomes of individuals of a given individual or species concerning morphology and number of each chromosome or pair of homologous.
Cell Nucleus Review - Image Diversity: karyotype
Chromosomes contain genes (genetic information in the form of nucleotide sequences) that command the protein synthesis thus regulating and controlling the activities of the cell. In the nucleus of somatic cells of diploid beings every chromosome has its correspondent homologous chromosome, both containing alleles of the same genes related to same functions. This occurs because one chromosome of one pair comes from the father and the other comes from the mother of the individual. The chromosomes that form a pair with alleles of the same genes are called homologous chromosomes. In humans, there are 22 pairs of homologous chromosomes plus the pair of sex chromosomes (the sex chromosomes are partially homologous).
The only human cells that do not have homologous chromosomes are the gametes since during meiosis the homologous chromosomes are separated.
Primary constriction is the narrower region of a condensed chromosome where the centromere, the structure that unites identical chromatids, is located. Secondary constriction is a region similar to the primary constriction, narrower than the normal thickness of the chromosome too, and in general it is related to genes that coordinate the formation of the nucleolus and control the ribosomic RNA (rRNA) synthesis. For this reason the secondary contrictions (that can be one or more in chromosome) is called nucleolus organizer region (NOR).
The chromosome region where the centromere is located is called primary constriction. In microscopic view this region is narrower (a stricture) than most part of the chromosome.
According to the position of the primary constriction the chromosomes are classified as telocentric, acrocentric, submetacentric or metacentric.
12. What is the structure that maintains identical chromatids bound?
The structure that maintains identical chromatids bound is the centromere.
Cell Nucleus Review - Image Diversity: centromere
Chromatin is a set of filamentous DNA molecules dispersed in the karyoplasm forming euchromatin and heterochromatin portions. Each chromatin filament is a complete chromosome (a DNA molecule, or double helix). The chromatin of the human somatic cell is formed by 46 DNA molecules (22 homologous chromosomes and 1 pair of sex chromosomes).
In interphase the cell prepares itself for division and duplication of DNA molecules occurs. The duplication of every DNA molecule forms two identical DNA double helix bound by a structure called centromere. In this phase each identical chromosome of these pairs is called chromatid. It is also during the interphase that the chromatids begin to condensate assuming the thicker and shorter shape typical of chromosome illustrations. So the phase of the cell cycle in which DNA duplicates is the interphase.
Some Biology textbooks call chromosome an unique filament of chromatin as well as the condensed structure made of two identical chromatids after the DNA duplication. Rigorously the pair of identical chromatids bound in the centromere are two copies of the same chromosome and therefore they are two identical chromosomes (and not only one).
Cell Nucleus Review - Image Diversity: chromatids
In the interphase there is intense metabolic activity in the cell nucleus: DNA is duplicating, euchromatin is being transcript and RNA is produced.
Every filament of chromatin is a complete DNA molecule (a complete double helix), i.e., a complete chromosome. A DNA molecule may form euchromatin and heterochromatin portions thus both are part of chromosomes.
Cell Nucleus Review - Image Diversity: chromosome structure
16. What are heterochromatin and euchromatin?
Chromatin is uncondensed nuclear DNA, the typical DNA morphology in interphase (the phase of the cell cycle in which the cells is not dividing itself). In this phase of the cell cycle chromatin can be found as heterochromatin, more condensed and dark (in electronic microscopy) portions of DNA molecules, and as euchromatin, less condensed and lighter portions of DNA molecules.
Since it is uncondensed the euchromatin is the biologically active portion of the DNA, i.e, the region that has active genes to be transcripted into RNA. The heterochromatin represents the inactive portions of the DNA molecule.
Cell Nucleus Review - Image Diversity: heterochromatin euchromatin
17. Of which substances is chromatin made?
Chromatin is made of DNA molecules associated to proteins called histones.
Cell Nucleus Review - Image Diversity: chromatin
18. Do all eukaryotic cells have nucleus and only one nucleus?
There are eukaryotic cells without nucleus and others with more than one nucleus. Osteoclasts, the cells responsible for resorption of the osseous matrix, for example, are multinucleate cells; striated muscle fibers are multinucleate too. Red blood cells are example of enucleated specialized cells.
Cell Nucleus Review - Image Diversity: cell nucleus miltinucleate cells enucleated cells
19. How are cells with delimited nucleus called? What are the main elements of the nucleus?
Cells with delimited nucleus are called eukaryotic cells. Organisms composed of one or more eukaryotic cells are called eukaryotes.
The mains elements of the nucleus are the chromatin (made of DNA molecules), the nucleolus, the karyolymph, or nucleoplasm, and the nuclear membrane (or karyotheca).
20. What are some biological examples in which lysosomic enzymes play fundamental role?
The remodelation of the osseous tissue, the function of acrosomes in sperm cells and the elimination of the tadpole tail are examples of biological processes in which lysosomic enzymes are key factors.
The bone is a tissue made of osteoblast-containing matrix (osteoblasts are the secretory cells of the osseous matrix), osteocytes (mature bone cells) and osteoclasts (the remodeling cells). Osteoclasts are responsible for the the continual renovation of the osseous tissue since their lysosomic enzymes digest the osseous matrix.
The sperm acrosome, for carrying digestive enzymes within, is responsible for the perfuration of the egg cell membrane in the fertilization process. The acrosome, located in the anterior end of the sperm cell, is a specialized region of the Golgi apparatus that accumulates great amount of digestive enzymes.
In tadpoles the tail regresses while the organism develops into an adult frog. This tissue destruction is a digestion of the tail own cells and extracellular materials and it is made by lysosomes and their enzymes. The complete digestion of a cell by its own mechanisms is called autolysis, a type of apoptosis (celll suicide).
Autophagic intracellular digestion is the cellular internal digestion of waste and residual materials. In general it is done by lysosomes.
Autophagic intracellular digestion is intensified in situations of starvation because in such condition the cell tries to obtain from its own constituent materials the nutrients necessary to stay alive.
22. What is heterophagic intracellular digestion? How is this process accomplished?
Heterophagic intracellular digestion is the breaking into smaller substances of external substances engulfed in the cell by pinocytosis or phagocytosis. Phagosomes or pinosomes fuse with lysosomes making the digestive vacuoles. Within the digestive vacuoles the molecules to be digested are hydrolyzed and the products of the digestion cross through the membrane and reach the cytoplasm or they are kept inside the vacuoles. The vacuole with residues from digestion is called residual body and by exocytosis it fuses with the plasma membrane and liberates its “waste” in the exterior space.
The organelles responsible for intracellular digestion are the lysosomes. Lysosomes are vesicles that contain digestive enzymes capable of breaking big molecules into smaller ones. These vesicles fuse with others that carry the material to be digested and then digestion takes place.
Cell Digestion Review - Image Diversity: lysosomes
24. What is intracellular digestion?
Intracellular digestion, or cellular digestion, is the breaking in the interior of the cell of big molecules coming from outside or even from the own cell metabolism into smaller molecules. Products and residues of the intracellular digestion are used by the cell or excreted.
Intracellular digestion is classified into two types: heterophagic intracellular digestion and autophagic intracellular digestion.
25. What is extracellular digestion?
Extracellular digestion is that in which food breaking into utile molecules that can be internalized by the cell is done in the extracellular space, i.e., outside the cell. In extracellular digestion the cells secret substances that break big molecules into smaller ones in the external environment. Later the cell can benefit from these products of the digestion.
26. What are some examples of secretory cells?
Endocrine and exocrine pancreatic cells, thyroid and parathyroid endocrine cells, adenohypophysis, adrenal and pineal endocrine cells, the many types of gastric exocrine and endocrine cells, the mucous secretory cells of the lungs and of the bowels, the salivary gland cells, the lacrimal gland cells, the sebaceous gland cells, the secretory cells of the ovaries and testicles, etc., are all examples of secretory cells.
The rough endoplasmic reticulum has in its outer membrane numerous ribosomes, structures where translation of messenger RNA and protein synthesis occur. These proteins are stored in the rough endoplasmic reticulum and later they go to the Golgi apparatus. Within the Golgi apparatus proteins are chemically transformed and when ready they are put inside vesicles that detach from the organelle. These vesicles fuse with the plasma membrane (exocytosis) in the right place and its content is liberated outside the cell.
28. Which cell organelles are well-developed in secretory cells?
In secretory cells, like the secretory cells of endocrine glands, organelles related to production, processing and “exportation” of substances are widely present and well-developed. These organelles are the rough endoplasmic reticulum and the Golgi apparatus.
The nuclear membrane of the secretory cells generally has more pores to allow the intense traffic of molecules related to protein synthesis between the cytoplasm and the nucleus.
Rough endoplasmic reticulum Golgi apparatus
29. What is meant by cellular secretion?
Cell secretion is the elimination to the exterior of substances produced by the cell (for example, hormones, mucous, sweat, etc.)
Cyclosis is a type of internal cell movement in which an oriented flow of circulating material is created and maintained in the cytoplasm by the action of microfilaments. Cyclosis is more easily observed in plant cells.
Cytoskeleton and Cell Movement - Image Diversity: cyclosis
31. What are some examples of movement created by the contraction of sarcomeres of the muscle cells?
The handling of a cup of coffee, the peristaltic movements of the bowels, the cardiac beats and even a smile are examples of movement created by contraction of the sarcomeres of the muscle cells. This contraction is a type of cell movement.
Amoeboid movements are created by cytoplasmic movements and plasma membrane projections called pseudopods. Their formation actively changes the external shape of some portions of the cell surface making it to move along a substratum. Pseudopods appear from differences of viscosity among neighboring regions of cytoplasm near the plasma membrane and from the contractile action of microfilaments.
Amoeboid movements occur, for example, in amoebas (a protozoan), organisms that use their movement to find food. The leukocytes, cells of the immune system, when attracted by chemical substances (immune mediators) use amoeboid movements to get out from capillaries in regions of tissue damage to participate in the inflammatory process.
Cytoskeleton and Cell Movement - Image Diversity: pseudopods
Cilia and flagella are structures found in some prokaryotes as well in some eukaryotic cells. They play defense, nutrition and movement roles for the cell. In eukaryotic cells of protists and animals they originate from centrioles that migrate towards the plasma membrane and differentiate into structures projected outside the cell. Each cilium or flagellum is made of nine peripheral pairs of microtubules and one central pair all covered by membrane. (In bacteria, flagella are made of a protein named flagellin and there can also be fimbria made of pilin.)
In the fixation base of each cilium or flagellum in the plasma membrane there are proteins that work as molecular motors providing movement for these structures with energy spending. Due to this energy spending ciliated or flagellated eukaryotic cells have a large number of mitochondria.
In humans ciliated cells can be found, for example, in the bronchial and tracheal epithelium. In these tissues the cilia have the defensive function of sweeping mucous and foreign substances that enter the airways. Sperm cells are typical example of flagellated cells their flagellum is the propulsion equipment for the movement towards the ovule.
Cytoskeleton and Cell Movement - Image Diversity: ciliated cell flagellate cell
34. What are cell movements? How are these movements created?
Cell movements are movements performed by cell structures, like the movements of cilia and flagella, the pseudopod movements (in amoeba, macrophages, etc.), the cyclosis of the cytoplasm and the sarcomere contraction in muscle cells.
Cell movements can be created by the citoskeleton action, by differences of viscosity among cytoplasmic regions and by intracellular contraction systems.
Microfilaments are made of actin (a protein). The contractile association of actin with myosin and other cytoplasmic proteins give to microfilaments the ability to promote cell movement.
Cytoskeleton and Cell Movement - Image Diversity: microfilaments actin and myosin intermediate filaments
