Chapter 1: The Genetic Information PDF

Localisation and nature of genetic information Pr. Wafae Benhardouze Learning outcomes In the end of this chapter you should be able to know: - Where is the genetic material stored ? - How does the genetic material transmited ? - What’s the chemical nature of the gen...

Localisation and nature of genetic information Pr. Wafae Benhardouze Learning outcomes In the end of this chapter you should be able to know: - Where is the genetic material stored ? - How does the genetic material transmited ? - What’s the chemical nature of the genetic material? Introduction -The reproduction leads to new generation of the same specie. -Each specie has structural and functional characteristics that are due to a hereditary program that is moving from a generation to an another: it’s called a hereditary material or a genetic material. - When appropriate conditions are available, the microscopic living organisms (bacteries…) reproduce by conserving the same hereditary material. I - Which part of the cell contains the genetic information ? 1 - Experiments on the unicellular organisms: Acetabularia + Acetabularia definition - It is a genus of green algae found in the Mediterranean and other warm seas (specially in subtropical waters). - It is an unicellular organism. - It is composed of three segments:. The "foot" or rhizoid which contains the nucleus. The "stalk,". The "cap.“ - The length of its stem “stalk” is between 3 and 5 cm. The diameter of its cap is 1 cm. - There are two different species of acetabularia: A.mediterranea and A. crenulata. - Each Acetabularia specie has a particular cap morphology. A. mediterranea has a smooth, disc shaped cap, while A. crenulata has a branched, flower-like cap. A. crenulata. A. mediterranea  + Cutting and transplantation nucleus experiments of acetabularia: Hämmerling experiments - Experiment 1: Cutting experiment " Hämmerling cut A. mediterranea and A. crenulata into two parts: the basal part include the nucleus and apical part without the nucleus. The basal part that include the nucleus lived and renovates the apical part, while the part that doesn’t include the nucleus dies. - Experiment 2: transplantation experiment The nucleus from A. crenulata is implanted into an enucleated part of A. mediterranea from which the cap has also been removed, the resulting cell will eventually develop a cap of the A. crenulata type. ►We deduce that the nucleus controls the form of cap 2 - Experiments on the multicellular organisms + Transplantation experiment on the toad  We observe that the toad resulting by cloning tooks the characters of the toad that we took its nucleus.  We deduce that the nucleus controlled the toad characters. conclusion - The nucleus controls the functions of the cells and therefore the characters of the individuals - The nucleus of a cell contains the genetic information that directs cellular development.  3 – Ultra structure of nucleus Remark: Plant cell contains several similar elements to the animal cell with some differences They are generally small in size They are usually larger than animal cells Cell wall is absent Cell wall is present Vacuoles are small in size Vacuol is larger in size Chloroplast is absent Chloroplast is present Cell is round and irregular in shape Geometric shape The nucleus is the largest cellular organelle ( specially in animal cell). In mammalian cells, the average diameter of the nucleus is approximately 6 micrometres (µm), which occupies about 10% of the total cell volume. It appears as a dense, roughly spherical or irregular organelle. The viscous liquid within it is called nucleoplasm, it contains a dense network of filament ; called chromatin (becomes chromosome). The nuclear envelope, otherwise known as nuclear membrane, consists of two membranes: an inner and an outer nuclear membrane. The space between membranes is called the perinuclear space.  II-How is genetic information transmitted from generation to the next? Remark: A zygote: is a cell formed by a fertilization phenomenon between two gametes (the sperm and the ovule). The zygote's nucleus is a combination of two nuclei ( of sperm and ovule gametes). The zygote's nucleus contains all of the genetic information necessary to form a new individual The zygote is the earliest developmental stage. The zygote undergoes successive divisions (mitosis division) to give 2 cells then 4;8;16;32……So it consists a cellular tissue what leads to the constitution of the embryo. The cell’s reproduction is biological phenomenon called mitosis  1- The role of mitosis in the transmission of the genetic information from cell to other 1.1- Study of mitosis in plant cell Activity 1 The following figures show the microscopic observation of mitosis division in onion root tip 1- What do you observe? 2- Deduce the mitosis phases  1.2- Mitosis phases characteristics  Prophase: is the first phase of mitosis. - The chromatin condenses into double rod-shaped structures called chromosomes in which the chromatin becomes visible. This process, called chromatin condensation. The chromosome is formed by two sister chromatids and they are attached to each other at a centromere. - The nuclear envelope has disappeared. - The spindle apparatus begins to extend outward from each of the two centrosomes (just for the animal cell).  Metaphase: Is th second phase of mitosis. - spindle fibers (which are microtubules) attach themselves to the centromere of each chromosome. Specifically, the connection is to specialized regions called kinetochores within the centromeres.  Anaphase: The thierd phase of mitosis - Chromosomes are split into two chromatids - The both sister chromatids of each chromosome are pulled apart by the spindle and dragged by their kinetochores toward opposite poles of the cell, that is, toward the opposite centrosomes.  Telophase is the forth and final phase of mitosis. - The chromosomes are at the poles and the nuclear envelope forms around each set of chromosomes to separate them from the cytoplasm. - The chromosomes are becoming more diffuse ( transformation of chromosomes to chromatin). - The duplicated genetic material carried in the nucleus of a parent cell into two identical daughter cells. Note 1.3- Mitosis definition Mitosis is a type of cell division (specially the somatic cells) in which one cell ( mother cell) divides into two cells (daughters cells) that are exactly the same. Each daughter cell has the same number of chromosomes and the same nature of genetic materiel as the original cell (mother cell) 2- The chemical nature of the hereditary material (genetic information) : 2.1- Griffith’s experiments Activity 2: Do the exercise Griffith's experiment, reported in 1928 by Frederick Griffith. Griffith used two strains of pneumococcus (Streptococcus pneumoniae) bacteria which infect mice – a type III-S (smooth) which was virulent, and a type II-R (rough) strain which was nonvirulent. The III-S strain covered itself with a polysaccharide capsule that protected it from the host's immune system, resulting in the death of the host, while the II-R strain did not have that protective capsule and was defeated by the host's immune system. 1- Analyse the obtained results 2- What do you conclude ?  First experiment : The injection of the mouse by the living smooth staphylococcus (S).  → The mouse died.  - Living (S) is considered mortal such that it causes a pneumonia which kills the mouse.  - Second experiment : Injection of the mouse by the living rough coccus (R) → The mouse survives.  Third experiment : After the heating of the living (S) it died ; then the mouse get injected by the dead coccus (S). → The mouse survives.  Fourth experiment : The injection of the mouse by the dead coccus “doesn’t die “ with the living coccus (R).  → Even if the mouse died.  ○ Griffith’s deduces that the death of the mouse was due to the presence of the living coccus (S) in its blood ; so that occur a convented factor which transformed living (R) non mortal to living (S) mortal.. What is the converted factor ?  2.2 –Experiment of Avery and his colleagues Activity 3 Questions: 1- Analyse the Avery experiment 2- What do you deduce?  Answers  1- We observe that a pure extract of the "transforming principle" was unaffected by treatment with protein-digesting enzymes or by RNA digesting enzymes, but was destroyed by a DNA-digesting enzyme.  So the converted factor is found on the chromosome is a Deoxyribo Nucleic Acid ; this is because the parts of DNA of dead (S) where automatically incorporated in the DNA of living (R) and it gets a new character. The formation of the capsule as well as it became living (S): It’s the phenomenon of bacterial transformation such that the living (R) transformed to living (S) and so on it will be able to change the hereditary material and it will be genetically converter.  2-We deduce that the genetic transformation is due by DNA  2.3 – Relationship between chromosomes and DNA Activity 4: Do the exercise  So much information is stored in DNA that it needs to be a very long molecule. Although only 2nm wide, the total length of DNA in the 46 chromosomes of an adult human cell is about 1.8 metres. This has to be packed into a nucleus which is only 6 μm in diameter. This is the equivalent of trying to get an 18 km length of string into a ball which is only 6 cm in diameter!  According to the previous information and the figure below, determine the relationship between DNA and chromosome  Answers  The DNA is wound around the outside of protein molecules called Histones ( histones are basic proteins. Because they are basic, they can interact easily with DNA, which is acidic).  The nucleosome is cylindrical in shape, about 11 nm wide by 6 nm long. It is made up of eight histone molecules. The DNA is wrapped around the outside of the cylinder, before linking to the next nucleosome.  The DNA between the nucleosomes is also held in place by a histone molecule. Nucleosomes line up like a string of beads to form a fibre 10nm wide ( chromatine). This string can be further coiled and supercoiled, involving some non-histone proteins.  The combination of DNA and proteins is called chromatin.  Chromatin is at its most condensed in chromosomes at the metaphase stage of mitosis: Chromosomes are made of chromatin  3 - cellular cycle concept  Activity 5: Do the exercise The DNA quantity is determined in nucleus of animal cell during different phases of the cell life. The table bellow shows the obtained results 1- Draw the curve: f(t)= DNA quantity 2- Show on the curve the different phases of cell life (- interphase : Germination 1:G1 or first gap, Synthesis: S, Germination 2: G2 or second gap – miotosis: P, M, A, T) and determine the duration of each phase 3- Analyze the obtained curve  3- The cell cycle is the regular sequence of events that takes place between one cell division and the next. It has two phases, namely interphase and mitosis ( nuclear division and cell division or cytokenesis). - Interphase: the cell grows and makes a copy of its DNA. It’s divided into three steps: + G1 phase: During G1also called the first gap phase, the cell grows physically larger, and makes enzymes and other proteins needed for growth and for later steps + S phase: S stands for synthesis (of DNA). This is a relatively short phase. The cell synthesizes a complete copy of the DNA in its nucleus. + G2 phase: During the second gap phase, or G2, the cell grows more, makes proteins and organelles, and begins to reorganize its contents in preparation for mitosis. G2 phase ends when mitosis begins. The G1, S, and G2 phases together are known as interphase. The prefix inter- means between, reflecting that interphase takes place between one mitotic (M) phase and the next. - Mitosis: During the mitotic (M) phase, the cell separates its DNA into two sets and divides its cytoplasm, forming two new cells.  III- Mechanism of DNA replication 1– The structure of DNA  Activity 6: By using figures below give: - Characterstics of DNA - Basic components of DNA Answers 1- DNA exists as a pair of molecules rather than a single molecule. These strands are entwined in the shape of a double helix and the helix is kept stable by hydrogen bonds, which can be found between the bases attached to the two strands. - The two strands of DNA run in opposite directions to each other( one strand from 3’ to 5’, other strand from 5’ to 3’) and are thus antiparallel direction. 2- The basic components of DNA are: +Phosphoric acid +Sugar +Nitrogenous bases A nucleotide = phosphate + a sugar + a nitrogenous base DNA is a polymere, made of units called nucleotides (or mononucleotides) Note Nucleoside: Sugar+base Nucleotide: Sugar+ phosphate+base  Conclusion  Deoxyribonucleic acid (DNA) IS the genetic information, is a polymere made of nucleotides units. Each nucleotide formed by: -1 Phosphoric acid: Gives a phosphate group. 2- Sugar: Deoxyribose Which is a cyclic pentose (5-carbon sugar). Carbons in the sugar are noted from 1' to 5'. A nitrogen atom from the nitrogenous base links to C1' (glycosidic link), and the phosphate links to C5' (ester link) to make the nucleotide. The nucleotide is therefore: phosphate - C5' sugar C1' - base. Deoxyribose 3-Nitrogenous bases: Are aromatic heterocycles; there are purines and pyrimidines. - Purines: Adenine (A) and Guanine (G). - Pyrimidines: Cytosine (C) and Thymine (T) - There are 3 hydrogen bonds betwen C and G bases , and just 2 hydrogen bonds betwen A and T bases  2- Mecanism of DNA replication 2.1- DNA replication concept DNA replication is the biological process by which DNA makes a copy of itself during cell division (during S phase):producing of two identical replicas of DNA from one original DNA molecule. This process occurs in all living organisms and is the basis for biological inheritance. 2.2- Models of DNA replication There were three basic models for DNA replication that had been proposed by the scientific community after the discovery of DNA's structure. These models are illustrated in the diagrams below:  Question: which one of the three models of DNA replication is correct? 2.3- Studying the Meselson and Stahl experiment Acitivity 7 The fellowing figure shows the Meselson and Stahl experiment 1- Explain the results obtained in the First generation 2- What do you conclude about the mode of DNA replication 3- What would you expect to see in the first generation if the DNA was replicated conservatively 4- What well be the result that are you expect in the third generation  Answers 1- After transferring the bacteria from medium that has 15N (heavy nitrogen) to the medium that has the 14N (The light nitrogen), we observe in the first generation ( after 20 min), that 100% of DNA stays in the midle of the tube, that means this DNA is intermediate: has two type of the nitrogens heavy and light: hybride DNA. One strand of DNA (the origin strand) has 15N while the new strand has the 14N( nitrogen existing in the new medium) 2- Because the DNA of all bacteria ( 100%) in the first generation is hybridn, so during the replication of DNA in the interphase, there is using of the nitrogen existing ine the medium ( the light nitrogen) to form the new strand of the DNA; as the result the DNA in the first generation has the original strand with the 15N and the new strand with 14N: at in fact the replication of DNA is semi-conservatively 3- if the DNA is replicated conservatively, we will obtained two bands in the tube 2: one in the bottom that represent the heavy nitrogen andthe other band in the top of tube that represent the light nitrogen: so we will be obtained 50% the bactreria have the DNA with 14N and 50% of bacteria have the DNA with 15N 3.4- Mecanism of DNA replication  Activity 8  1-Watch the video  - By exploiting the video informations  2- Determine differents kind of molecules needed to the DNA replication  3- Determine the role of each molecule  4- By using the text below, give the appropraite order of the DNA replication’s steps 5- Basic concepts in genetics - Gene : basic unit of genetic information. Genes determine the inherited characters. - Genome :the set of genetic information( gene set) -Chromosomes :storage units of genes. - DNA :is a nucleic acid that contains the genetic instructions specifying the biological development of all cellular forms of life - Exemple: Human genome - Locus: location of a gene/marker on the chromosome. -Allele: one variant form of a gene/marker at a particular locus.  A dominant allele : is expressed even if it is paired with a recessive allele.  A recessive allele: is only visible when paired with another recessive allele.  Genotype: At each locus (except for sex chromosomes) there are 2 genes. These constitue the individual’s genotype at the locus. Phenotype: The expression of a genotype is termed a phenotype. For exemple, hair color, weight, or the presence or absence of a disease Genotype phenotype Example: genotypes Eb- dominant allele. Ew- recessive allele.