1 Genes Are DNA

Ex Biochem c1-genes DNA Introduction Figure 1.2

Ex Biochem c1-genes DNA Polynucleotide Chains Nitrogenous Bases 鹼基 Linked to a Sugar – Phosphate Backbone A nucleoside consists of a purine or pyrimidine base linked to position 1 of a pentose sugar.

Ex Biochem c1-genes DNA 4 Transfection DNA can enter eukaryotic cells and produce functional proteins Become part of the genome DNA can also be introduced into eggs by microinjection Become part of the genome

Ex Biochem c1-genes DNA 5 Nucleic acid structure Positions on the ribose ring are described with a prime (′) to distinguish them. The difference between DNA and RNA is in the group at the 2′ position of the sugar. DNA has a deoxyribose sugar (2′ – H) RNA has a ribose sugar (2′ – OH) A nucleotide consists of a nucleoside linked to a phosphate group on either the 5′ or 3′ position of the (deoxy)ribose. Successive (deoxy)ribose residues of a polynucleotide chain are joined by a phosphate group Between the 3′ position of one sugar and the 5′ position of the next sugar One end of the chain (left) has a free 5′ end The other end has a free 3′ end

Ex Biochem c1-genes DNA 6 Nucleosides Nucleoside: Nucleoside: a compound that consists of D- ribose 核糖 or 2-deoxy-D-ribose 去氧核糖 bonded to a nucleobase by a  -N-glycosidic bond

Ex Biochem c1-genes DNA 7 Nucleotide Nucleotide: Nucleotide: a nucleoside in which a molecule of phosphoric acid is esterified with an -OH of the monosaccharide, most commonly either the 3 ’ -OH or the 5 ’ -OH

Ex Biochem c1-genes DNA 8 Nucleotides Deoxythymidine 3 ’ -monophosphate (3 ’ - dTMP)

Ex Biochem c1-genes DNA 9 DNA Structure

Ex Biochem c1-genes DNA 10 Pyrimidine/Purine Bases

Ex Biochem c1-genes DNA 11 Other Bases Several “ unusual ” bases occur, principally but not exclusively, in transfer RNAs

Ex Biochem c1-genes DNA 12 Figure 1.07: A polynucleotide has a repeating structure.

Ex Biochem c1-genes DNA 13 DNA Structure Writing a DNA strand an abbreviated notation even more abbreviated notations: pdApdCpdGpdT, or pdACGT, or ACGT

Ex Biochem c1-genes DNA DNA Is a Double Helix The B-form of DNA is a double helix consisting of two polynucleotide chains that run antiparallel. The nitrogenous bases of each chain are flat purine or pyrimidine rings They face inward They pair with one another by hydrogen bonding to form A-T or G-C pairs only

Ex Biochem c1-genes DNA 15 Figure 1.08: The double helix has constant width.

Ex Biochem c1-genes DNA 16 Figure 1.09: Flat base pairs connect the DNA strands.

Ex Biochem c1-genes DNA 17 The diameter of the double helix is 20 Å There is a complete turn every 34 Å Ten base pairs per turn The double helix forms: a major (wide) groove a minor (narrow) groove Figure 1.10

Ex Biochem c1-genes DNA 18 DNA double helix

Ex Biochem c1-genes DNA DNA Replication Is Semiconservative The Meselson – Stahl experiment used density labeling to prove that: The single polynucleotide strand is the unit of DNA that is conserved during replication Each strand of a DNA duplex acts as a template 模版 to synthesize a daughter strand.

Ex Biochem c1-genes DNA 20 Figure 1.11: Base pairing accounts for specificity of replication. DNA replication is semiconservative

Ex Biochem c1-genes DNA 21 Semiconservative Replication

Ex Biochem c1-genes DNA 22 Enzymes The enzymes that synthesize DNA are called DNA polymerases (DNA 聚合脢 ) The enzymes that synthesize RNA are called RNA polymerases Nucleases are enzymes that degrade nucleic acids They include DNAases and RNAases They can be divided into endonucleases and exonucleases.

Ex Biochem c1-genes DNA 23 Figure 1.14: Endonucleases attack internal bonds. Figure 1.15: Exonucleases nibble from the ends.

Ex Biochem c1-genes DNA Genetic Information Can Be Provided by DNA or RNA Cellular genes are DNA Viruses and viroids may have genomes of RNA DNA is converted into RNA by transcription RNA may be converted into DNA by reverse transcription The translation of RNA into protein is unidirectional. Figure 1.16

Ex Biochem c1-genes DNA 25 Figure 1.18: Genomes vary greatly in size.

Ex Biochem c1-genes DNA Nucleic Acids Hybridize by Base Pairing Heating causes the two strands of a DNA duplex to separate. The T m is the midpoint of the temperature range for denaturation. Complementary single strands can renature when the temperature is reduced. Denaturation and renaturation/hybridization 雜交 can occur with the combinations: DNA – DNA DNA – RNA RNA – RNA They can be intermolecular or intramolecular

Ex Biochem c1-genes DNA 27 Figure 1.20: DNA can be denatured and renatured.

Ex Biochem c1-genes DNA Mutations Change the Sequence of DNA All mutations 突變 consist of changes in the sequence of DNA. Mutations may: occur spontaneously be induced by mutagens Figure 1.22

Ex Biochem c1-genes DNA Mutations May Affect Single Base Pairs or Longer Sequences A point mutation changes a single base pair. Point mutations can be caused by: the chemical conversion of one base into another mistakes that occur during replication Insertions are the most common type of mutation They result from the movement of transposable elements

Ex Biochem c1-genes DNA 30 A transition replaces a G-C base pair with an A-T base pair or vice versa. Figure 1.23Figure 1.24

Ex Biochem c1-genes DNA The Effects of Mutations Can Be Reversed Forward mutations inactivate a gene Back mutations (or revertants) reverse their effects Insertions can revert by deletion of the inserted material Deletions cannot revert Suppression occurs when a mutation in a second gene bypasses the effect of mutation in the first gene. Figure 1.25