Biology 1-H Study Guide Chapter 7 Structure of DNA Polymer of nucleotides Nucleotide: 5-carbon sugar (deoxyribose) Phosphate group Nitrogenous base Nitrogenous bases: Purines: adenine and guanine Pyrimidines: thymine and cytosine Individual nucleotides are joined together to make a long chain o Phosphate group of one nucleotide is joined to the sugar of another nucleotide to make the chain. Rosalind Franklin Experiment · Used purified DNA in a long glass tube so that most of the DNA strands were parallel. · Shot the tube with x-ray beams to create an image on film o Particles of x-rays diffract off the structure of DNA to produce an image which is captured on film. · Look at Pg 142 figure 7-8 o What does it look like? What does the x-ray represent? Why? Model of DNA – when twisted and compressed, and look at head on, it looks like an X. Gave clues to the structure of DNA. The regular interval of the black dots signify that whatever makes up DNA occurs at regular intervals. It also signified that two stands make up the structure of DNA. Watson and Crick’s Discovery · Began making models of what they thought DNA looks like when they saw Franklin’s x-ray picture. It triggered a connection in their head and within weeks they figured out the correct structure of DNA. · Franklin’s x-ray suggested 2 strands which were twisted. Watson and Crick made the structure appear like a ladder which was twisted like a spiral staircase. With this structure, they could account for the x-ray picture of Franklin’s. · They called their structure a double helix which allowed for the nitrogenous bases to be paired up to hold the rungs of the ladder together. They later discovered that hydrogen bonds hold the bases together. · Another interesting pattern arose – A always paired with T and C always paired with G. · The attraction between the bases is strong – called base pairing. The Genetic Code and Experiments DNA is the genetic code – the way cells store the program which gives full instructions on how to function. o Must be easily replicated and past onto future generations How did we discover the idea of DNA? How was the molecule of DNA first thought of? 1928. Frederick Griffith Studied bacteria which caused pneumonia Had 2 strains: 1 which caused the disease to develop and looked smooth in the petri dish and 1 which did not cause pneumonia and looked rough in the petri dish. Griffith performed a series of experiments where he injected mice with combinations of the bacteria. · Smooth bacteria – mouse died · Rough bacteria – mouse lived · Took the smooth bacteria (caused disease) and killed them with heat. When injected, the mouse lived. o What does that tell you? If the bacteria was dead, the disease could not be passed onto the mouse. · Injected mice with the mixture of heat-killed bacteria and some rough, harmless bacteria. What do you think happened? Mice died. Why? Somehow the heat-killed bacteria had transferred the disease-causing agent into the harmless strain of bacteria. When Griffith cultured the bacteria from the mice, they were smooth. He believed that one strain of bacteria transformed into the other – called the process transformation. Next thing to discover? What had actually passed between the strains of bacteria? 1944 – Avery, McCarty and MacLeod made an extract (a juice) of the heat-killed bacteria. · They treated the bacteria with enzymes that destroy carbs, lipids and proteins. The transformation still occurred. · They treated the bacteria with RNA and DNA eating enzymes. The transformation did not take place. 1952 -- Alfred Hersey and Martha Chase · Investigated viruses which attack bacteria – bacteriophages · Virus contains DNA surrounded by a protein coat. They land on the surface of the bacteria, inject their contents and use the bacteria for the replication of the virus DNA. The bacteria bursts, releasing all of the newly formed viruses. · They did a simple but effective experiment to determine whether the DNA, the protein coat or both were injected into the bacteria for replication. · Labeled the protein coat with radioactive sulfur-35. · Labeled the DNA with radioactive phosphorous-32. · Radioactive isotopes are used as tracers to follow the track of a molecules when they undergo chemical reactions · After mixing the labeled viruses with the bacteria, only the radioactive phosphorous-32 was found showing, therefore only the DNA was injected into the bacteria. · Because after the DNA was injected, replication takes place, it was assumed that the DNA is the main molecule of genetic information. DNA Replication Structure of DNA is a double helix – with special base pairing A – T and G – C 2 strands of DNA are said to be complimentary. If you know the order of the nucleotides of one of the strands, you can figure out the order of the nucleotides on the other strand. Because of this it is very easy to picture replication. · Separate the two strands of DNA and use each one as a template to create a new opposing strand. · The double helix unwinds and unzips breaking the hydrogen bonds between the bases and separating the strands. · Once opened, enzymes go to work to match loose nucleotides to the original strands of DNA making two new complete strands of DNA. Section 7.2 RNA – ribonucleic acid Acts as a messenger between DNA and ribosomes Structure: Similar to DNA 3 parts: o phosphate group o 5 – carbon sugar o nitrogenous base adenine, guanine, cytosine and uracil · Uracil takes the place of thymine and is found only in RNA. It base pairs with A as T would. · Similar structure as thymine but is missing a methyl group: RNA is made to be a messenger (mRNA) - used to carry message from nucleus (DNNA) to the ribosome– made during process called transcription. Transcription: a molecule of DNA is copied into a complimentary strand of RNA. Why must we use mRNA? DNA cannot leave the nucleus and proteins must be made on the ribosome – need some way to the get the info from the nucleus to the ribosome. Transcription occurs similar to DNA replication. · Double helix unzips and a complimentary strand of RNA is synthesized by an enzyme called RNA polymerase. · RNA polymerase attaches to the DNA strand at a special "start" sequence and begins to make the strand of RNA until it reaches the end. Section 7.3 DNA signals the production of proteins (made of amino acids) 20 amino acids · if 1 nucleotide codes for an amino acid there would be only 4 amino acids · if 2 nucleotides code there would be only 16 amino acids · if there were 3 nucleotides – how many amino acids would code? 64 (43) Amino acids are coded by three nucleotides read in a sequence. · The packet of 3 nucleotides is called a codon. · Each amino acid has more than 1 codon except for methionine – only 1. Methionine is used as the codon which signals the start of protein synthesis. Translation: process of translating the language of RNA into protein. · Before the polypeptide can be synthesized there is an intermediate step using another form of RNA · tRNA (transfer RNA) – compliment of mRNA, still using uracil in place of thymine. Structure of tRNA – separate pieces of single stranded RNA which attach to mRNA and have an amino acid attached. Compliment nucleotides are called an anticodon. The translation from mRNA tRNA protein happens on the ribosome. Biology 1-H Study GuideChapter 8 Section 8.1 Cell Growth Living things grow because cells need to divide to create other cells. Organisms do not grow because their cells get exceedingly large.Why can’t cells grow to very large sizes?o Recall surface area to volume ration. Surface area needs to be appropriately large to allow all of the needed nutrients into the cell. Certain cells can grow at astonishing rateso E. coli can grow and form new cells in 20 – 30 minutes under ideal conditions. Ideal conditions are not present all the time so E. coli doesn’t grow to catastrophic proportions o In one day a single bacteria, under ideal conditions, can grow to have a mass of 14 kilograms. Controls of cell growth:o Use tissue culture: place cells in a petri dish with a nutrient broth and the cells will divide and multiply until they cover the bottom of the dish. Once the entire surface of the dish is covered, the bacteria stop dividing. o Density dependent inhibition: cells will not grow if the amount of cells around them reach or exceed the threshold amount. The cells need to be in direct contact with the nutrient source therefore they will not grow in multiple layers. Cancer Cells:o Cancer cells have abnormal cell cycles which allows them to continuously divide. o They are not density dependent and will continue to grow even when many cells are present. o Scientists believe that there are checkpoints within the cell cycle, at the end of G1 and G2. These checkpoints are either absent or mutated in cancer cells which allows them to pass thru the checkpoints and grow uncontrollably. Section 8.2 Mitosis and CytokinesisChromosomes: structures which contain genetic informationProkaryotes: double stranded circular DNAEukaryotes: distinct lengths of DNA which is highly organizedChromosomes are made of chromatin – a complex of DNA and proteins (page 165 in book)o DNA double helix is wound around histone proteins to make "beads" called nucleosomes o Long strands of nucleosomes are wound together and tightly coiled o The coils are then coiled into a super coil which compacts them to fit into the size of a chromosome o Condense 10,000X to fit onto a chromosome During cellular division a chromosome consists of 2 sister chromatids (identical lengths of chromatin) joined together by a centromere.Cell cycle: period from 1 cellular division to the next2 distinct phases: Interphase and MitosisInterphase:o 90 % of the cell cycle o 3 distinct phases · G1 phase (gap phase 1): the cell grows to make more room for the expanding chromosomes and to prepare to divide · S phase: the DNA (in the form of chromatin) is replicated so that each chromosome has an identical twin, to which it is attached · G2 phase (gap phase 2): the second growth phase where organelles and materials needed for division are replicated. The cell readies itself for division. Page 167 Figure 8 – 10 diagram of the cell cycle and the relative lengths of the phasesMitosiso Only 10% of the cell cycle o 4 distinct phases, they occur in a series with 1 phase not being able to occur until the previous phase has been completed finished. Prophase: longest phase (50 – 60 % of mitosis)o Chromatin begins to condense into chromosomes (remember that there are 2 sister chromatids in one chromosome) o Centrioles appear and separate to opposite ends of the cell called poles. The centrioles contain microtubules from the cytoskeleton which will guide the movement of the chromosomes within the cell. o Centrioles send out spindle fibers which attach to the centromeres of the sister chromatids. o The nuclear membrane breaks down and the nucleoli (where ribosomes are made) breaks down. Metaphase: shortest phase of mitosiso Microtubules from the spindle fibers move and align the chromosomes along the center of the cell. o Array of microtubules is called as aster because it looks like a star Anaphase:o The spindle fibers which are connected to the chromosomes begin to contract and pull the chromosomes to opposite sides of the cell. o The chromosomes, now single, are pooled at the poles of the cell. Telophase:o Chromosomes at the poles begin to decondense and uncoil to their original state o A nuclear envelope reforms around the chromatin and the spindle breaks apart. Cytokinesis:o Divides the cell into two identical daughter cells by splitting the cytoplasm and dividing the cell membrane o The microtubules left at the center of the cell form a "drawstring" which pinches the cytoplasm of the cell until the cell splits into two.
