 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

 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

 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

 Nitrogen Bases with a double ring structure  Adenine and Guanine

 Nitrogen bases with a single ring structure  Cytosine, Thymine, and Uracil

 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)

 In a single nucleotide, the 1 st carbon of the pentose sugar is covalently bonded to the nitrogenous base (glycosyl bond)  The 5 th carbon of the pentose sugar is covalently bonded to the phosphate group (ester bond)  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.

Nucleotides are joined together by phosphodiester bonds and form the DNA backbone

 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

Glycosyl Bond Ester Bond Phosphodiester Bond Hydrogen Bond

 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

 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)

 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 1.8m in length  If fits into a cell because it is very tightly packed – which also keeps it organized!

 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

 It takes 200 nuleotides to form a nucleosome  The histone are positive, the DNA is negative – so they are strongly attracted!

 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

 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

 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.

 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.

 For life to perpetuate, cell must replicate (undergo cell division – mitosis and cytokinesis)  Before mitosis can occur, the DNA in the nucleus must duplicate

 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)

The enzyme DNA helicase unwinds the double helix and separates the complimentary strands by breaking the hydrogen bonds between them.

 Base pairs have a natural propensity to anneal (stick together)  Single-stranded binding protein (SSBs) bind to the exposed DNA single strands and block hydrogen bonding  DNA gyrase relieve tension produced by the unwinding of DNA

 DNA will only replicate small segments of the larger strand at one 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

 In eukaryotes, five different types of DNA polymerase are present  The enzyme that builds the complementary strand using the template strand as a guide is DNA polymerase III

DNA polymerase III cannot initiate a new complementary strand by itself The enzyme primase creates an RNA primer – which is 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

 Once the RNA primer is in place, DNA polymerase III can start elongation by adding free deoxyribonucleotide triphosphates to the growing complementary strand

 The new strand is made up of “free floating nucleotides” or deoxyribonucleoside triphosphates that are found in the nucleus

 DNA polymerase III uses the energy derived from breaking the bond between the first and second phosphate to drive the condensation reaction that adds a complementary nucleotide to the elongating strand  The extra two phosphates are recycled by the cell and are used to build more nucleotide triphosphates

 DNA is always synthesized in the 5’ -3’ direction  Since DNA strands run antiparallel, only one strand is able to be built continuously  The strand which is able to be continuously synthesized in the 5’ to 3’ direction is called the leading strand

 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 ( nucleotides in length) are produced called Okazaki fragments  (at the beginning of each Okazaki fragment there will be a RNA primer)

 The enzyme DNA polymerase I removes the RNA primers from both the leading and lagging strands and replaces them with the appropriate DNA nucleotides.

 On the lagging strand, the enzyme DNA ligase will attach the Okazaki fragments of the lagging strand together.  As the 2 new double strands of DNA are made, they will automatically twist into a helix.

 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.

 hill.com/sites/ /student_view0/c hapter14/animations.html# hill.com/sites/ /student_view0/c hapter14/animations.html#

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