Lecture 6: Mitosis and Apoptosis Fall 2024 PDF

Lecture 6: Mitosis and Apoptosis Fall 2024 Textbook sections: 9.3, 10.1, 10.2 and 10.3 Last class We examine the catabolism of glucose; how are cells able to breakdown and extract energy from glucose. Today we will look at an anabolic process…mitosis. Characteristics of life One...

Lecture 6: Mitosis and Apoptosis Fall 2024 Textbook sections: 9.3, 10.1, 10.2 and 10.3 Last class We examine the catabolism of glucose; how are cells able to breakdown and extract energy from glucose. Today we will look at an anabolic process…mitosis. Characteristics of life One of the characteristics of life is growth, reproduction and development. In each of those cases, the division of cells is required. Cell division is an anabolic process. https://vimeo.com/315487551 Cell division Depending on the organism, cell division can be used for growth, reproduction or both. Sexually reproducing species generally use cell division for growth Asexually reproducing species use cell division for growth and reproduction Cell division Fundamentally, cell division is concerned with the duplication and segregation of genetic material into two daughter cells.  Segregation of DNA The point of cell division is to produce two daughter cells each containing the same DNA as the parent cell This is more complex than it sounds, as DNA is a complicated molecule that requires organization DNA is long: in E. coli, DNA is 500 times longer than the cell itself! In humans, the DNA found in one cell is about 2m long! DNA must therefore be organized to fit into cells In bacteria: DNA is organized as a single circular chromosome, or single strand of DNA It is compacted and organized into the nucleoid region of the cell with the help of the structural maintenance of chromosome or SMC protein Eukaryotic DNA In plants and animals, the organization of DNA is much more complex, as there is more DNA Most of the DNA is found in the nucleus of the cell, most eukaryotes possess multiple chromosomes, or strands of DNA, all the DNA in an organism is known as a genome Within the nucleus each chromosome is organized into chromatin, a loosely packed complex of DNA and protein Chromatin contains about 40% DNA and 60% protein Chromatin organization In the nucleus, DNA wraps around proteins called histones, a DNA-histone complex is called the nucleosome Nucleosomes can then wrap around each other into an even more compact structure called a solenoid This is the usual (non dividing) state of chromatin During division chromatin assembles into distinct chromosomes During cell division, scaffold proteins allow for an even larger level of compaction At this point, the chromatin changes into a distinct mitotic chromosome Chromosomes are different Different species have a wide range of chromosomal numbers Humans have a total 46 chromosomes, organized into 23 almost identical pairs  Chromosome numbers Each organism is defined by a certain number of chromosomes The haploid number of a species (n) is the number of distinct chromosomes necessary for the definition of the species In humans, the diploid (2n) number reflects the total number of chromosomes in the cell, it reflects the contribution from each parent Different species have different ploidy numbers. For example, some plants, such as Brussel sprouts are tetraploid (4n). Meaning they have four copies of each distinct chromosome. Karyotypes Besides differing in numbers between species, chromosomes vary in size and shape within the same species This array of chromosomes is called a karyotype What are the haploid and diploid numbers for humans based on this karyotype? In humans we can see that each chromosome is present twice. Why is this? Chromosomal anatomy Each paternal and maternal chromosome are considered to be homologs…they essentially encode the same traits (except for sex chromosomes). Both chromosome 1, for example, are considered homologous chromosomes  Chromosomal anatomy Each homologous chromosome consists of a centromere and a kinetochore The centromere is a point of constriction of the chromosome that contains repetitive sequences that will bind specific proteins The kinetochore is a disk of such proteins that bind microtubules which will separate the chromosomes during cell division  When cells divide, DNA doubles If the point of cell replication is to produce daughter cells that each contain genetic information identical to that of the parent cell, DNA must replicate. When it does, chromosomes become arranged into pairs of sister chromatids Although the amount of DNA in a pair of sister chromatid is twice that of a regular, unreplicated chromosome, a pair of sister chromatids is still referred to as a single chromosome Now that we know how DNA is organized in prokaryotes and eukaryotes, how does cellular replication take place in those organisms? Eukaryotic cell division In eukaryotic cells, cell division and DNA replication are two distinct processes To get a better understanding of division in these organisms it is important to consider the whole reproductive/life cycle of the cell, known as the cell cycle Eukaryotic cell cycle The eukaryotic cell cycle is divided into five phases who belong to two distinct portions of the cycle  Interphase Interphase is considered as a period of growth and consists of: G1: Gap 1 phase, primary growth phase of the cell. Longest phase for most cells S: Synthesis phase, replication of DNA occurs during this phase G2: Gap 2, second growth phase, preparation for separation of newly replicated genome, duplication of centrioles, synthesis of tubular apparatus  M phase M phase is considered to be the dividing phase of the cell, it consists of: Mitosis: phase during which replicated chromosomes are separated into daughter cells, consists of 5 stages Cytokinesis: process by which the cytoplasm divides and leads to the generation of two new daughter cells Cell cycle The length of the cell cycle spends on the cell type examined Typical mammalian cell = 24 hrs (M=1hr) Embryo=20 mins Fruit fly embryo= 8 mins G0 G0 is known as a resting state, its length varies widely depending on cell type Some cells may remain in G0 for days, months or years Others enter G0 permanently  M phase Contains 5 phases: Prophase, Prometaphase, Metaphase , Anaphase, Telophase  Prophase: formation of mitotic apparatus Chromosomes condense from chromatin into sister chromatids Nuclear envelope breakdown, nucleolus disappears ER and Golgi disperse Assembly of mitotic spindle: centrioles move to the poles of the cell, forming an axis of microtubules, called the spindle apparatus The tubules associated to the centrioles are known as an aster, not found in plant cells  Prometaphase: Chromosome attachment Occurs after total disassembly of nuclear envelope Microtubules attach to the kinetochore at the centromeres of sister chromatids Each sister chromatid is attached to a microtubule from a different pole of the cell Chromosomes move towards the centre of the cell Metaphase: Chromosomes align at the equator of the cell This is called the metaphase plate (not an actual structure)  Anaphase Sister chromatids separate and are pulled apart Removal of centromere proteins (cohesins) that hold sister chromatids together Microtubules attached to kinetochore start pulling the sister chromatids to opposite poles of the cell Cell poles start to separate  Telophase Spindle apparatus disassembles Nuclear envelope forms around newly separated chromosomes Nucleolous reappears Cytoplasmic organelles reasserted to areas that will separate into new cells Cytokinesis Separation of cytoplasm and cytoplasmic components In cells without cell walls, a belt of actin pinches the cell, creating a cleavage furrow As the constriction increases, the cleavage furrow deepens until there are two cells Cytokinesis The cell wall in plants is too tough to be pinched by actin filaments Instead, cell membrane and wall components are assembled as vesicles perpendicular to the spindle apparatus, until the interior portion of the cell membrane is reached This growing portion is called the cell plate Cytokinesis Recap https://www.youtube.com/watch?v=f-ldPgEfAHI Cell division Cell division is tightly controlled in organisms Loss of cell cycle control results in dangerous diseases such as cancer. The cell cycle contains specific checkpoints that allow for its regulation, and the verification of proper cell function  Apoptosis When checkpoints are not passed (ie. There is a problem with the cell), apoptosis is triggered. Apoptosis is also known as programmed cell death. Apoptosis involves the coordinated destruction of the cell and cell components Apoptosis is also used during development for the removal of unneeded cells. Apoptosis Apoptosis is also important during development, as it is used to remove unneeded cells Homework Quiz on Friday Lab on Wednesday Test September 30th.