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Biology - Chapter 8 notes.docx

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Western University
Biology 1201A
Richard Gardiner

Biology: Exploring the Diversity of Life Chapter 8: Cell Cycles (PAGES 161 - 177) What do all cells require in order to survive? - A complete set of genetic instruction (coded in DNA cells) - To produce required molecules - Direct life processes Why do cells divide? - Growth - Repair - Development Key Points of the Cell Cycle - Activities of a cell from one cell division to the next o Cell Grows adding more cytoplasmic constitutes o DNA is replicated o Cell divides into two identical daughter cells  Has transmitted a complete copy of genetic information The cell Cycle of Prokaryotic Organism - Mechanism of Prokaryotic cell division is known as binary fission o Period B – A cell grows for some time before entering DNA synthesis o Replication of Prokaryotic chromosome commences at the origin of replication (ori) – in the middle of the cell where the enzymes for DNA replication are located o Period C – Prokaryotic chromosome is replicated, the Cell elongates and the chromosomes are separated to opposite ends of the cell o Period D – The plasma membrane pinches together between them and two daughter cells are formed The Cell Cycle of a Eukaryotic Organism - Cell division of somatic (non-sex) cells is called mitosis Eukaryotic Chromosome o Contains almost all genetic information (mitosis only deals with chromatin [nuclear chromosomes])  Mitochondria and Chloroplasts also has DNA o Chromosomes – Long thread-like structures (Linear DNA)  DNA molecules + Associated protein that stabilizes the DNA, assist in packages DNA during cell division and influences the expression of individual genes o Human Cells:  Most eukaryotes have two copies of each chromosome in their nuclei (pl. for nucleas)  Diploid vs. Haploid  Diploid just means that the cell contains two sets of DNA ("di" means two). Most cells in the body are diploid. The diploid number of chromosomes in a human somatic (body) cell is 46.  Haploid means that the cell contains one set of DNA, half of a diploid cell. The haploid number of chromosomes in a human body cell is 23. A human haploid body cell denotes a sex cell (i.e. sperm, egg).  This doesn't mean that the haploid cell doesn't have all the chromosomes- they do; it's just that a full set of DNA is 23 chromosomes; we get 2 sets total, one from mom and one from dad. We do need all 46 chromosomes... it's just that for producing gametes (sex cells) the number needs to be halved because when a sperm fertilizes the egg, the two sets of DNA come together in the zygote.  Ploidy: refers to the number of pairs of chromosomes in a cell o Chromosome vs Chromatid vs. Chromatin  Chromatin is the combination of all the DNA and its associated proteins in the nucleus. It gets condensed into chromosomes during mitosis.  After DNA replication, your chromosomes have identical Sister Chromatids. A chromosome that looks like the typical "X" has two chromatids held together at a region called the centromere.  In late anaphase, the chromatids have been pulled apart to opposite poles of the cell. Each half of the cell now has a set of chromosomes that are made of one chromatid each.  So a chromosome that has been pulled apart in anaphase is still a chromosome, it just has one chromatid instead of two. o DNA replication increases the amount of DNA in the nucleus but does not increase the number of Chromosomes - Nucleic Acids o Two types: DNA (Deoxyribonucleic acid, stores hereditary information) and RNA (Ribonucleic Acid – carries out functions)  All nucleic acids are polymers of nucleotides; consists of three parts (1) Nitrogenous base (ring of carbon and nitrogen atoms) a. Pyrimidine – one carbon-nitrogen ring b. Purines- two rings (2) A five-carbon sugar a. Differ only in the chemical group bound to the 2’ carbon (3) 1-3 phosphate groups o Nucleotide are linked by bridging phosphate group between the 5’ carbon of one sugar and the 3’ carbon of the next sugar [Known as a phosphodiester bond] o Note: the nucleotide thymine (T) in DNA is not found in RNA, it is replaced with uracil(U) o DNA takes form of a double helix (i.e. double stranded)– two nucleotide chains  Connected by a pair of nitrogenous bases [Hydrogen bonded] held perpendicular to the helical axis  Purine-Pyrimidine Pairs: Adenine with Thymine (A-T) [two h-bonds] and Guanine with Cytosine (G-C) [three h-bonds]  Note: in RNA thymine changes to Uracil, so Adenine with Uracil (A-U) - Cell Cycle o Formation of a new daughter cell marks the beginning of a cell cycle, Interphase:  Grows and replicates it DNA in preparation for mitosis  DNA uncondensed (chromatin)  Nucleus defined, centrosomes replicated, microtubules extend from centrosomes  During the initial growth stage, called t1e G phase of the cell cycle, the cell makes various RNAs, proteins, and other molecules but not nuclear DNA (the G in 1 stands for gap, referring to the absence of DNA synthesis) and the size increases  S phase initiates when DNA replication synthesis begins
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