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Chapter 5 Cell Function Notes

by: Tiffany Norris

Chapter 5 Cell Function Notes Biol 201

Marketplace > Loyola Marymount University > Biology > Biol 201 > Chapter 5 Cell Function Notes
Tiffany Norris
Loyola Marymount University
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About this Document

Material will be on Exam 2, as well as Quiz 2.
Cell Function
Yiwen Fang
Class Notes
Cell, Biology, DNA




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This 7 page Class Notes was uploaded by Tiffany Norris on Sunday September 25, 2016. The Class Notes belongs to Biol 201 at Loyola Marymount University taught by Yiwen Fang in Fall 2016. Since its upload, it has received 92 views. For similar materials see Cell Function in Biology at Loyola Marymount University.

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Date Created: 09/25/16
Cell Function Quiz 2: Chapter 5 Study Guide [DNA & CHROMOSOMES] Leftover Questions: 1. For a DNA with 100 nucleotides long, how many possible sequences are there? 100 a. 4 possible nucleotides, so 4 2. For a protein that is 100 AAs long, how many possible sequences are there? a. 20 possible amino acids, so 20 10 DNA in Humans • Each human contains 2 meters of DNA, cramped into a 5-8 µm nucleus • Humans’ DNA is separated into a set of chromosomes (23 different types of chromosomes) o We are diploid, i.e. one of each chromosome type from mom and one from dad. o Each chromosome consists of a single, long, linear DNA molecule folded into a compact structure (the chromosome) • We have 3 billion base pairs and 30,000 genes st • Down syndrome is when someone has three copies of the 21 chromosome o Fetuses usually don’t survive when there are defects in chromosomes, but some do, e.g. people with down syndrome DNA Structure • Nucleotides are composed of a nitrogen-containing base and a five carbon sugar, which is attached to one or more phosphate groups. o Sugar – deoxyribose o Bases – adenine (A), guanine (G), cytosine (C), thymine (T) o Bases are complementary* § A always pairs with T § G always pairs with C § T & C are pyrimidines § G & A are purines • Each polynucleotide strand can be thought of as a necklace o Four types of beads = the four bases A, G, C &T o Sugar-phosphate backbone = the string • The DNA is polar because of the way the nucleotide subunits are linked together o 5’ end = the end with the free phosphate group ß START o 3’ end = the end with the free hydroxyl group (OH) ß END • Shape of DNA is an anti-parallel* double helix* o Each helix is held together by hydrogen bonds between the bases on each strand o Called a “right hand helix” § There are about 10 bases on each turn on the helix § There is a major groove and a minor groove (see figure). Compacting DNA: • DNA uses histones to fold and compact • It is necessary to compact DNA because it creates a protective structure and allows for easier division 1. The most abundant chromosomal proteins are the histones, which pack DNA into a repeating array of DNA–protein particles called nucleosomes. a. The nucleosome core consists of a complex of eight histone proteins—two molecules each of histones H2A, H2B, H3, and H4 (the nucleosome is shown as the yellow bead in the figure below) b. Histones are positively charged 2. Nucleosomes pack together, with the aid of histone H1 molecules, to form a 30-nm fiber. 3. This fiber is coiled and folded, producing a more compact chromatin structures. Called scaffolding, to give the final compaction. THE 4 LEVELS OF COMPACTION (1) NUCLEOSOMES (2) FIBER, “BEADS ON A STRING” (3) FILAMENT (4) SCAFFOLDING Chromosome Structure & Function Chromosomes are ONLY PRESENT WHEN CELLS DIVIDE Chromosomes are composed of DNA + protein • Diploid- 2 sets of chromosomes (2 copies for each gene) 
 • Haploid- 1 set of chromosomes 
 • Homologous chromosomes (homologs)- similar chromosomes, i.e. paternal vs. maternal chromosomes 
 • The Karyotype is the visual representation of all of the chromosomes someone has. o Seeing them all lined up helps us to see when there is an obvious defect. • Chromosome function: § Chromosomes must replicate, therefore they have multiple origins of replication that direct the duplication of the DNA § § Centromeres act as handles during cell division § Telomeres are at each end of the chromosome for protection • Chromatin makes up the chromosome. Structure is dynamic. There are two types: 1. Euchromatin § Most genes in Euchromatin are turned ON and active § More sensitive to nuclease enzyme § Less compact than heterochromatin § Stains lighter under electron microscope 2. Heterochromatin § Stains darker under electron microscope § Found during the M-phase in the cell cycle § Most genes are turned OFF and inactive because it is so tightly packed *see section on chromosome regulation* Chromosome Structure Regulation How does the cell expose a localized region of DNA when it is all coiled up, so that it can be used in transcription or regulation? • Chromatin-remodeling complexes: use energy (ATP) to reposition the portion of DNA around nucleosomes by modifying histone proteins with acetyl, phosphate and methyl groups ⇒ This affects the affinity and stability of the chromatin fiber ⇒ And allows the histone proteins to function as docking sites for regulatory proteins to work on. • Modifying histones helps to ⇒ Maintain chromatin structure and . . . ⇒ Helps to extend the state of the chromatin i. Heterochromatin is highly condensed a. the pattern of histone tail modification causes the DNA to become so highly compacted that the packaged genes cannot be expressed to produce RNA and protein. 
 ii. Euchromatin is less condensed and therefore supports gene expression iii. Propagation of chromatin: Euchromatin modified to become heterochromatin, therefore turning OFF gene expression. Both eu & hetero chromatin are products of histone modification • Chromatin structure can be transmitted from one cell generation to the next, producing a form of epigenetic inheritance that helps a cell to remember the state of gene expression in its parent cell • Epigenetics: changes in a chromosome’s structure, NOT the DNA sequence The Cell Cycle a series of events that take place in a cell as it grows and divides There are two main phases (1) Interphase (2) M-Phase (Mitosis) I. Interphase a. Chromosomes are not lined up b. Not heavily condensed yet c. But are not totally randomly distributed either i. Some chromosomes are attached to a particular site on the nuclear envelope. They like to hang out there. Genes • A gene is a piece of DNA that encodes a RNA and/or Protein • Gene expression is the process of going from DNA to RNA to Protein • Gene dosage: Random inactivation of one X chromosome in females GLOSSARY Chromatin: Complex of DNA, histones, and nonhistone proteins found in the nucleus of a eukaryotic cell. The material of which chromosomes are made. Chromosome: Consists of the DNA + histones. Compact structure of DNA Chromatin: Part of chromosome Diploid: A cell or organism containing two sets of homologous chromosomes and hence two copies of each gene or genetic locus. DNA (deoxyribonucleic acid) molecule: Consists of two long polynucleotide chains. Each chain (“strand”) is composed of four types of nucleotide subunits. The strands are held together by hydrogen bonds between the nucleotides, while the sugar in the nucleotides are held together by phosphodiester bonds. DNA (in general): Stores genetic material. Discovered by scientists Watson, Crick, Franklin & Wilkins. Gene Expression: The process by which a gene makes its effect on a cell or organism by directing the synthesis of a protein or an RNA molecule with a characteristic activity. Gene: Region of DNA that controls a discrete hereditary characteristic of an organism, usually responsible for specifying a single protein or RNA molecule. Haploid: A cell or organism with only one set of chromosomes, as in a sperm cell or a bacterium. Homologs: One of the two copies of a particular chromosome in a diploid cell, one from the father and the other from the mother. Hybridization: Experimental process in which two complementary nucleic acid strands form a double helix; a powerful technique for detecting specific nucleotide sequences. Karyotype: A display of the full set of chromosomes of a cell arranged with respect to size, shape, and number. Q & A [?] How did we discover that DNA is the source of genetic material? [A] Experiment by Griffith in which mice were infected with pneumonia bacteria (two strains, one harmless, one deadly). Found that the harmless bacteria were converted to lethal bacteria which posed the question, “how was this change passed on to progeny bacteria?” [?] What is the Virus Cocktail Story? [A] Scientists Hershey and Chase did an experiment to further prove that DNA is genetic material. The researchers were studying T2—a virus that infects and eventually destroys the bacterium E. coli. The beauty of T2 is that these viruses contain only two kinds of molecules: DNA and protein. The experiment was fairly straightforward. Because the viral DNA enters the bacterial cell, while the rest of the virus particle remains outside, the researchers decided to radioactively label the protein in one batch of virus and the DNA in another. Then, all they had to do was follow the radioactivity to see whether viral DNA or viral protein wound up inside the bacteria. To do this, Hershey and Chase incubated their radiolabeled viruses with E. coli; after allowing a few minutes for infection to take place, they poured the mix into a blender. The blades cut the empty virus heads from the surfaces of the bacterial cells. The researchers then centrifuged the sample to separate the heavier, infected bacteria. They found that the radioactive DNA entered the bacterial cells, while the radioactive proteins remained with the empty virus heads. This experiment demonstrated conclusively that viral DNA enters bacterial host cells, whereas viral protein does not. Thus, the genetic material in this virus had to be made of DNA. They used found that radioactive DNA enters the bacterial cells, while the radioactive proteins remain with the empty virus heads. They found that the radioactive DNA was also incorporated into the next generation of virus particles. This experiment demonstrated conclusively that viral DNA enters bacterial host cells, whereas viral protein does not. Thus, the genetic material in this virus had to be made of DNA.


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