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Published byClaire Richard Modified over 9 years ago
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3.1 & 7.1 3.4 & 7.2
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Genetic information is stored in molecules called nucleic acids. There are 2 types of nucleic acids DNA: deoxyribonucleic acid ◦ Double stranded RNA: ribonucleic acid ◦ Single stranded
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Nucleotides are the building blocks of nucleic acids A single nucleotide consists of: ◦ A pentose sugar ◦ A phosphate group ◦ A nitrogenous base Components are held together with covalent bonds
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Also known as an organic base, or a nitrogen base 5 different bases: ◦ Adenine (A) ◦ Cytosine (C) ◦ Guanine (G) ◦ Thymine (T) – only found in DNA ◦ Uracil (U) – only found in RNA
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Nitrogen Bases with a double ring structure Adenine and Guanine
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Nitrogen bases with a single ring structure Cytosine, Thymine, and Uracil
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In RNA, the sugar is “ribose” In DNA, the sugar is “deoxyribose” (The difference is the presence or lack of oxygen on the 2 nd carbon)
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In a single nucleotide, the 1 st carbon of the pentose sugar is covalently bonded to the nitrogenous base. The 5 th carbon of the pentose sugar is covalently bonded to the phosphate group The 3 rd carbon of the pentose sugar is covalently bonded to the phosphate group of the next nucleotide in the chain. This bond is called a phosphodiester bond.
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Remember DNA, is double stranded The 2 strands of DNA are complimentary The nitrogen bases of 2 complimentary nucleotides hydrogen bond to each other to create the double strand Adenine always bonds to Thymine and Cytosine always bonds to Guanine
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Covalent Bond Phosphodiester bond (Covalent Bond) Hydrogen Bond
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There are 2 H bonds between Adenine and Thymine There are 3 H bonds between Guanine and Cytosine How many would you expect uracil to make with a potential nucleotide? DNA forms a double helix The helix is created by H-bonds between non- consecutive nucleotides
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The 2 DNA strands will each have a phosphate at the end of one strand, and a sugar at the opposite end. The end that has a phosphate is referred to as the “5 prime end” (5’) The end that has a sugar is referred to as the “3 prime end” (3’) The 2 strands are ANTIPARALLEL (because their 3’ and 5’ terminals are at opposite ends)
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DNA is extremely long If you took the DNA of a single cell and stretched it out into one long double helix, it would measure 1.8 in length If fits into a cell because it is very tightly packed – which also keeps it organized!
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Just like thread is spun around a spool to keep it organized, DNA is coiled around a group of eight proteins called histones. The complex of histones and DNA is called a NUCLEOSOME
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It takes 200 nuleotides to form a nucleosome The histone are positive, the DNA is negative – so they are strongly attracted!
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Histone proteins: ◦ 8 histone proteins (4 types, 2 of each type) inside each nucleosome ◦ 1 histone protein outside each nucleosome, which functions to organize and hold the nucleosome together
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A series of nucleosomes coil into chromatin fibres The chromatin fibres then coil to form a supercoil The supercoiled chromatin is what makes up a chromosome A chromosome is one unbroken double- stranded DNA helix
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Not only do nucleosomes keep DNA organized, they also prevent trancription Transcription is when DNA is used as a template to produce an RNA strand. For this to occur, the enzyme RNA polymerase must attach to the 3’ end of a DNA strand.
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When DNA is organized in a nucleosome, the promoter region is inaccessble so transcription cannot take place When the cell requires transcription, enzymes will alter the shape of the nucleosome to allow RNA polymerase to attach.
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DNA contains genetic information But in actualilty only a small portion of DNA constitutes genes “Unique genes” / “Single-Copy genes” / Codable Genes” make up 1.5% of human genetic material. These are the genes that carry out genetic information.
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The remainder (and majority) of DNA are “repetitive sequences” that have no known function (non-coding regions) Since repetitive sequence vary from person to person, they are useful in DNA profiling, which allows for DNA fingerprinting to identify sample from individuals.
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In eukaryotic cells, many genes are discontinuous. A single gene is interrupted with a long non-coding sequence. EXON – the coding sequence INTRON – intervening, non-coding sequence
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For life to perpetuate, cell must replicate (undergo cell division – mitosis and cytokinesis) Before mitosis can occur, the DNA in the nucleus must duplicate
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DNA replication is semiconservative The parent double helix produces 2 daughter double helices. Each daughter molecule will have a parental strand and a daughter strand (an old strand and a new strand)
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The new strand is made up of “free floating nucleotides” or deoxyribonucleoside triphosphates that are found in the nucleus.
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1. The enzyme DNA helicase unwinds the double helix and separates the complimentary strands by breaking the hydrogen bonds between them.
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Single-stranded binding protein (SSBs) prevent the complimentary nitrogen bases from reforming their hydrogen bonds. DNA gyrase relieve tension produced by the unwinding of DNA
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DNA will replicate small segments of the larger strand at a time. So only small segments will be unwound and separated by helicase at any give time. These segments are called replication bubbles. The junction where the 2 strands are still attached is called the replication fork
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2. The enzyme primase creates an RNA primer – which is 10 - 60 RNA nucleotides. The RNA primer temporarily attaches to the 3’ end of a DNA strand. The purpose of the primer is to create a starting point for the DNA nucleotides to attach
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3. Starting from the RNA primer, the enzyme DNA polymerase III adds free floating deoxyribonucloside triphophates to the DNA strand using complimentary base pairing rules. The original parent strand acts as a template
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DNA is always synthesized in the 5’ -3’ direction. This means: a nucleoside that is being added will bond its phosphate group (at the 5’ end) to a nucleoside that is already apart of the strand. The next nucleoside will bond to the 3’ end of the previous nucleotide with its 5’ end.
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The strand that is built continuously and in the same direction of helicase is the leading strand
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The other strand is called the lagging strand. It is synthesized discontinuously in the direction away from the replication fork and in the opposite direction of helicase. As a result, short fragments (1000-2000 nucleotides in length) are produced called Okazaki fragments (at the beginning of each Okazaki fragment there will be a RNA primer)
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4. The enzyme DNA polymerase I removes the RNA primers from both the leading and lagging strands and replaces them with the appropriate DNA nucleotides.
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5. Enzyme DNA ligase will attach the Okazaki fragments of the lagging strand together. 6. As the 2 new double strands of DNA are made, they will automatically twist into a helix.
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DNA polymerase I and III “proofread” the newly created strands checking for mistakes. If there is a mistake, the enzymes act as an exonuclease It removes the incorrect nucleotide and replaces it with the correct one.
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http://highered.mcgraw- hill.com/sites/0072437316/student_view0/c hapter14/animations.html# http://highered.mcgraw- hill.com/sites/0072437316/student_view0/c hapter14/animations.html# (* show Replication Fork first)
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