Lecture 2 Our genetic material DNA chromosomes and genome PDF

BIOL1930: Genetics for Everyone Lecture 2 Our genetic material: DNA, chromosomes and genome Sunday and Wednesday @11:30 a.m. Dr. Eman AbdelAlim Rabie, PhD Biotechnology Fall 2024 Getting to know Genetics! Module I...

BIOL1930: Genetics for Everyone Lecture 2 Our genetic material: DNA, chromosomes and genome Sunday and Wednesday @11:30 a.m. Dr. Eman AbdelAlim Rabie, PhD Biotechnology Fall 2024 Getting to know Genetics! Module I: Basic concepts in Genetics! Module II: Genetic disorders, what could go wrong? Module III: Behavioral Genetics Module IV: The fascinating things Genetics can do! Contents 1. Properties of a genetic material. 2. DNA Structure: A Brief Review. 3. Organization of Human Chromosomes. 4. Organization of the Human Genome. 5. The relevance of mitosis and meiosis. 6. The cell cycle: mitosis and meiosis.  “Genetics is the study of genes, genetic variation, and heredity in living organisms.” Heredity (inheritance or biological inheritance) is the passing on of traits from parents to their offspring through reproduction. Through heredity, variations between individuals can accumulate and cause species to evolve. Variation between species Variation between individuals of the same species What does a genetic material have to possess? Four necessary properties of genetic material: 2. This 3. The 4. There must be the information information 1. The genetic potential for mutation must be used to contained must material must that can result in the direct the be transmissible encode physical variation that functioning of from cell to cell & information. exists among individuals cellular from generation & between species. processes. to generation. DNA, the double helix, where is the “code”?  DNA, the double helix, where is the “code”? Nucleotide = Deoxy ribose sugar (ring) → Polynucleotide strand → Two anti parallel polynucleotide strands → Double helix DNA + phosphate (grey ball) through + one nitrogen base (either A,C,G,T) “Base-pairing & Hydrogen bonds” DNA structure in words! DNA is a polymer of nucleotides. (polymer = multiple repeated units). Each nucleotide has three components: a five-carbon sugar ring called deoxyribose, a nitrogenous base (either A,C,G,T) and a phosphate group. The double helix (right-handed spiral staircase shape) has two polynucleotide chains run in opposite directions i.e. antiparallel, held together by hydrogen bonds between pairs of bases: A of one chain paired with T of the other, and G with C. Where is the “code”? The specific nature of the genetic information encoded in the human genome lies in the sequence of C’s, A’s, G’s, and T’s on the two strands of the double helix along each of the chromosomes. Because of the complementary nature of the two strands of DNA, knowledge of the sequence of nucleotide bases on one strand automatically allows one to determine the sequence of bases on the other strand. From DNA into chromosomes Each human chromosome consists of a single, continuous DNA double helix; therefore, each chromosome in the nucleus is a long, linear double-stranded DNA molecule. If the DNA from all 46 chromosomes in a human cell nucleus was laid out end to end, it would measure approximately two meters or 3 billion base-pairs; however, its diameter would be only 2 nm. Considering that the size of a typical human cell is about 10 µm (100,000 cells lined up to equal one meter), DNA must be tightly packaged to fit in the cell’s nucleus. “Size of Human DNA per cell is approximately 2 meters in length and size of nucleus diameter is 10 microns, basically a strand width of human hair is about 100 microns. So, 10 microns in diameter nucleus is packaging it’s 2,000,000 times length of DNA within its boundaries.” DNA organization into chromosomes DNA organization into chromosomes There are five levels of chromosome organization (from top to bottom): DNA double helix. Nucleosome = DNA double helix wrapped around eight proteins (histones). Nucleosomes wrapped into around many histones, creating the appearance of beads on a string. The fourth level is chromatin fiber (coils) and supercoils that further condenses into the final fifth level “chromosome.”  Human cell = 46 chromosomes = 23 pairs of homologous chr. = 22 pairs+XX (female) OR 22 pairs+XY (male) Autosomes: 22 pairs alike in males and females Sex chromosomes (XX females, XY males) are chromosomes which contain genes including those involved in sex determination. Homologous chromosomes: are members of a pair of chromosomes carry matching genetic information; therefore, they have the same genes in the same sequence. All chromosomal pairs are homologous except the XY pair in males since Y chromosome is shorter and carries different sequences (genes) than X chromosome.  Homologous chromosomes: are members of a pair of chromosomes carry matching genetic information; therefore, they have the same genes in the same sequence. At any specific locus, however, they may have either identical or slightly different forms of the same gene, called allele, e.g., allele for eye color could be brown, blue or green…etc. Figure 2-8 Size of the 24 human chromosomes. Chromosomes are ordered left to right by size. Size of each human chromosome, in millions of base pairs (1 million base pairs = 1 Mb). Organization of the Human Genome Each human chromosome consists of a single, continuous DNA double helix; that is, each chromosome in the nucleus is a long, linear double- stranded DNA molecule. The word “genome” is the total set of genetic material within the cell; in humans it describes the whole 46 DNA molecules, totaling more than 6 billion nucleotides or 3 billion bp (base-pairs). Figure 2-8 Gene content of the 24 human chromosomes. Chromosomes are ordered left to right by gene content. Two facts! Mitochondrial DNA is circular DNA and much smaller than the nuclear DNA within the chromosomes, it contains only 16,500 base pairs compared to over 3 billion pairs in the nuclear DNA. Inherited from mothers only! (Why?). Fact 2: Only 2% of your genome codes for proteins! Human genome Nuclear genome Mitochondrial genome 46 chromosomes Circular DNA not chromosomes Size: 3 billion bp or 3000 Mb Size: 16.5 Kb Number of genes: 25,000 genes Number of genes: 37 25% 75% Genes and other Repetitive DNA of sequences for gene unknown function regulation Protein coding DNA Non coding DNA 2% 23% Two types of cell division Two types of cell division Two types of cell division Mitosis, also called equational division Meiosis, also called reduction division or or asexual reproduction: is a process of sexual reproduction: is the process of cell duplication, in which one diploid division of a diploid germ cell (2n) cell (2n) divides into two genetically involving two fissions of the nucleus and identical diploid daughter cells (2n). giving rise to four haploid gametes/sex This means the cell’s chromosomes are cells (n), each possessing half the copied and then distributed equally number of chromosomes of the original between the two new nuclei of the cell. daughter cells. It occurs in germ cells to produce It occurs in somatic cells (all body cells gametes/sex cells (sperms and ova). except germ cells producing gametes). Why do we learn this? The relevance of mitosis and meiosis The biological significance of mitosis and meiosis lies in ensuring the constancy of chromosome number— and thus the integrity of the genome—from one cell to its progeny and from one generation to the next. The medical relevance of these processes lies in errors of one or the other mechanism of cell division, leading to formation of an individual or of a cell lineage with an abnormal number of chromosomes and thus abnormal dosage of genomic material. Studying meiosis makes us understand the basis of genetic variation via crossing over or recombination. Mitosis in 3D: M Phase of the Cell Cycle - YouTube The cell cycle**: Cell division Cell cycle has two major phases: the interphase and the mitotic phase. Note that Relaxed uncondensed DNA (called chromatin) is duplicated in S phase of the interphase BEFORE even mitosis starts! S phase= DNA Synthesis phase **The cell cycle is an orderly sequence of events that describes the stages of a cell’s life from the division of a single parent cell to the production of two new daughter cells. Please focus on what happens to chromosomes! Interphase Prophase Metaphase Anaphase Chromatin Note that chromatin condenses into Note that sister duplicated chromosome chromatids separate shape (X shape having into two daughter two sister chromatids) at chromosomes start of mitosis Telophase and Cytokinesis Cytokinesis: The division of the cytoplasm of a single eukaryotic cell into two daughter cells, it occurs after nuclear division during mitosis and meiosis. Meiosis in 3D Meiosis - YouTube Mitosis vs Meiosis Meiosis I: From (2n) to (n) Mitosis From (2n) to Meiosis II: (2n) From (n) to (n)  (n) (n) (2n) * Note that meiosis I, (1) homologous chromosomes exchange segments (CROSSING OVER) AND (2) homologous chromosomes separate, their sister chromatids remain attached!  (n) (n) (n) (n) (n) * (n) (n) Note that meiosis II, (n) sister chromatids separate into two daughter chromosomes similar to what happened in mitosis Please focus on what happens to chromosomes! Meiosis I: From (2n) to (n) * Meiosis II: * From (n) to (n)  Thank you! Instructor email: [email protected] TA: [email protected]