GENETICS. Chapter 1: Cell cycle. Thème 1 : La Terre dans l Univers A. Expression, stabilité et variation du patrimoine génétique.

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1 Introduction: GENETICS 3M = first look at genetics (study of inheritance, discovery of chromosomes, genes, dominant and recessive alleles and the DNA molecule within chromosomes) 2D = not much in fact, only the DNA structure (double helix, sugar-phosphate backbone, nitrogenous bases and specific pairing between A-T and C-G) and genetic code (a specific sequence of bases has a precise meaning different alleles come from small differences in the sequence) Chapter 1: Cell cycle Do you remember DNA file (virtual experiment) Intro DNA diagram Investigating mitosis (practical course) Looking for DNA in the cell cycle (practical course + diagrams of DNA coiling states) All living organisms need to produce new cells. They can only do this by division of pre-existing cells. The life of a cell can be thought as an ordered sequence of events, called the cell cycle. The cell cycle refers to the events between one cell division and the next. It can be roughly divided into interphase and cell division also called mitosis. Interphase is an active period in the life of a cell consisting in three phases, the G1 phase, S phase and G2 phase. During the S phase, the cell copies all genetic material so that after the mitosis both new cells have a complete set of genes. Each chromosome has one copy of the DNA at the beginning of the S phase and two identical at the end, called chromatids. The global DNA amount is doubled during the S phase. During the interphase, chromosomes are uncoiled so they can t be seen with an optical microscope. Mitosis consists in four phases, the prophase, metaphase, anaphase and telophase. In most of the cases, this division is a perfect reproduction which keeps all the caryotype characteristics (number and shape of the chromosomes). 1. Prophase: Chromosomes (two chromatids) are supercoiling and become visible in the cell with an optical microscope. The nuclear membrane breaks down and the chromosomes are everywhere in the cytoplasm. 2. Metaphase: Chromosomes are moved to the equator of the cell and aligned on the metaphase plate. 3. Anaphase: The pairs of sister chromatids separate and are pulled in opposite directions of the cell. Each pole of the cell gets exactly the same genetic material. 4. Telophase: Nuclear membranes reform around the new chromosomes uncoiling (one chromatid) at each pole. A new cell membrane forms between the two daughter cells: this last step is called cytokinesis. Meselson-Stahl experiment has brought an end to the debate regarding the DNA replication mechanism. Their results demonstrate that the only consistent hypothesis is the semi-conservative one. When DNA replicates, the two strands of the double helix separate. Each of these original

2 strands serves as templates for the creation of a new strand. The result will be two DNA molecules, both composed of original strand and a newly synthesized strand. The base sequence on one strand determines the base sequence on the other strand because the bases are complementary (A-T and G-C). From one DNA molecule (chromatid), a second one is created, identical to the original one. In prokaryotic cells, there is only one origin of replication whereas in eukaryotic there are many. Replication proceeds in both directions from the origin and on both strands. Figure: cell cycle DNA amount and DNA structure Figure: DNA replication Summary diagram on the cell cycle!

3 Chapter 2: Genetic variability and DNA mutations DNA replication limits (scientific article analysis) Mutations and genetic diversity (practical course) Mutations and environment (figures analysis) Genetic conditions (questions) DNA repair diagram and Genetics definitions below Nothing s perfect in life not even DNA replication. During this phenomenon, changes in DNA sequence can occur from time to time because of DNA polymerase errors. Those changes also happen spontaneously apart from replication. Mutagenic agents such as UV radiations, tobacco smoke or chemicals, increase the rate of errors. Most of the time though, the DNA repairing system fixes it. Very rarely, the repairing system fails and the error stays. This change is called mutation and can be transmitted, if it is not lethal, from cell to cells according to the cell cycle. In that case, there are two main possibilities: The mutation occurred in a somatic cell and so all the clones of this cell will be affected The mutation occurred in a germinal cell (that is making the gametes) and so it will become hereditary Mutations are not all bad. Without random mutations, there would be no evolution because they are the source of genetic diversity, the basis of all biodiversity! Definitions: Alleles = one of an number of different forms of gene Genotype = the alleles of an organism Phenotype = the characteristics of an organism Homozygous = having two identical alleles of a gene Heterozygous = having two different alleles of a gene Dominant allele = an allele that has the same effect on the phenotype whether it is present un the homozygous or heterozygous state Recessive allele = an allele that only has an effect on the phenotype when it is present in the homozygous state Co-dominant alleles = pairs of alleles that both affect the phenotype when in a heterozygote Gene mutation = a change to the base sequence of a gene Carrier = an individual that has one copy of a recessive allele that causes a genetic disease in individuals that are homozygous for the allele Blood type (exercise)

4 Chapter 3: From genotype to phenotype From genotype to phenotype (practical course) Sickle cell anemia and malaria (exercise) From DNA to proteins (practical course) RNA splicing figures From DNA to proteins 2 (practical course) I. What s phenotype really? Phenotype stands for the organism characteristics. Those characteristics may be spotted at different levels of organisation: the molecular one, the cellular one and the macroscopic one (i.e. organism). For example, sickle cell anemia can be depicted as: Chronic anemia with very painful crisis = ORGANISM Altered red blood cell (sickle ad not disc shapes) = CELLULAR Change in the 6 th amino acid of the β-globin forming the hemoglobin (a valine replacing a glutamic acid) = MOLECULAR All those levels are linked. Change in the protein sequence of hemoglobin leads to molecular long chains and thus to sickle red blood cells. Because sickle cells are not flexible, they get stuck into small blood vessels and bring pain and anemia. Phenotype also depends on the environment. People with sickle cell anemia may not live in high altitudes because the lack of oxygen there brings severe crisis. II. Proteins: between genotype and phenotype Proteins are made of amino acids. There are 20 different amino acids known by their 3 first letters ( val for valine, glu for glutamic acid, etc.). The 3D shape of the protein will depend on the length and sequence of amino acids. A change in DNA sequence (i.e. mutation) can lead to a modification of one amino acid in a protein (sickle cell example with a T replacing an A) and thus a change of the form and function of the protein. Genotype that gathers all DNA sequences, so all alleles of every gene in the caryotype, is responsible for the molecular phenotype, the proteins set present in one cell. Genes code for the production of proteins. III. From DNA to proteins A gene is a unit of heredity that consists of a sequence of DNA bases. This sequence of bases does not, in itself, give any observable characteristic in an organism. The function of most genes is to specify the sequence of amino acids in a particular protein. Two processes are needed to produce a specific protein : Transcription = RNA synthesis from DNA (in the nucleus of eukaryotic cells) Translation (traduction in French) = protein synthesis from RNA (in the cytoplasm)

5 Diagram of those two steps A. Transcription, first step of protein expression The synthesis of the messenger ARN uses DNA as a template. In fact, DNA molecule unzips thanks to an enzyme, the RNA polymerase. Then, free nucleotides are added to the RNA strand because they are complementary with bases of one of the DNA strands, called the transcribed strand. RNA sequence has only one strand and is complementary with DNA transcribed strand but Uracil replaces Thymine. One DNA sequence can be transcribed many times and give many mrna! Diagram of transcription B. RNA maturation In eucaryotes, the immediate product of mrna transcription is referred to as precursor mrna (or pre-mrna) as it must go through several stages of post-transcriptional modification to become mature mrna. One of these steps is called splicing (épissage in French). Some portions of premrna will be removed and won t contribute to the protein formation: the introns. The remaining coding portions, the exons, will be spliced together to form the mature RNA. Depending on the context (environment, cell location ), a pre-mrna can be spliced differently and thus different proteins can be obtained from the same gene. Figure: pre-mrna splicing in two different proteins (4 exons) C. Translation, from mrna to proteins In the cytoplasm of eukaryotic cells, mrna is translated into protein thanks to the genetic code, which relates a codon (three bases of RNA) to a particular amino acid. This genetic code is: Universal because all living organisms share it (despite a few exceptions) Redundant because different codons can give the same amino acid Many actors are needed for translation, different proteins and RNA (rrna for ribosomes, trna for the amino acid carrying). It consists in fact in three stages: initiation, elongation and termination. Diagram of translation (figure) Exercises: Under conditions where methionine must be the first amino acid, what protein would be coded for by the following mrna? -CCUCAUAUGCGCCAUUAUAAGUGACACACA- Which mrna codes for the following protein? Phenylketonuria (exercise) Met-Arg-Ser-Leu-Glu