nucleic acids

what are the 2 main types of nucleic acids?

DNA and RNA

what is DNA?

• deoxyribonucleic acid is an important information-carrying molecule

• it is the genetic material in all living organisms ranging from single celled bacteria to multicellular organisms

what does the DNA code do?

it tells us what to do and what proteins to make

what is the genome?

the cell’s entire genetic content

• the study of the genome is genomics

difference of DNA location in eukaryotes and prokaryotes?

• eukaryotes- DNA is found mainly in the nucleus, but there is also some in the chloroplasts and mitochondria

• prokaryotes- DNA is not enclosed in a membrane envelope

what is RNA?

• ribonucleic acid is similar to DNA

• it is mostly involved in synthesising proteins

what is formed from RNA?

ribosomes

• and proteins are the protein builders of the cell

role of mRNA?

• DNA never leaves the nucleus, so it uses messenger RNA (mRNA) to transfer genetic information to the rest of the cell

  • mRNA moves out of the nucleus to transfer information to the ribosomes.

  • the mRNA is then used to make proteins

components of nucleic acids?

DNA and RNA are made of monomers called nucleic acids

• nucleotides join together to form polynucleotides

• polynucleotides make up nucleic acids such as DNA or RNA

structure of nucleotides?

• each nucleotide is formed from a pentose sugar, a nitrogen-containing organic base and a phosphate group

what does a condensation reaction between 2 nucleotides form?

• a phosphodiester bond

  • these condensation reactions happen between a sugar group on one nucleotide and a phosphate group on a different nucleotide

components of a DNA nucleotide?

• a deoxyribose sugar

• a phosphate group

• an organic base- adenine, thymine, cytosine, guanine

components of an RNA nucleotide?

• a ribose sugar

• a phosphate group

• an organic base- adenine, uracil, cytosine, guanine

structure of DNA?

• a DNA molecule is a double helix with 2 polynucleotide chains held together by hydrogen bonds between specific complementary base pairs

• the sugar and phosphate lie on the outside of the helix and form the DNA’s backbone

• the nitrogenous bases are stacked on the inside of the helix, like a pair of staircase steps

• hydrogen bonds hold the base pairs together

  • RNA is only made of one polynucleotide

how many bonds are there between adenine and thymine, compared to cytosine and guanine?

• adenine and thymine- 2 hydrogen bonds

• cytosine and guanine- 3 hydrogen bonds

base pairing rules in DNA:?

• adenine can pair with thymine

• cytosine can pair with guanine

  • this means that there is always an identical number of adenine and thymine bases and of guanine and cytosine bases in DNA

• strands are complementary to each other

types of bonding in RNA and DNA?

• DNA: base pairing holds the 2 DNA polynucleotides together by hydrogen bonding

• RNA: the single polynucleotide chain is held together by phosphodiester bonds between the nucleotides

why are the 2 polynucleotide strands antiparallel?

because they run in opposite directions

what happens when a cell divides?

• it forms 2 daughter cells

  • this means that before cell division, the cell must duplicate its DNA so that each daughter cell can inherit the full set of DNA

DNA replication?

• DNA is made of 2 polynucleotide chains that form a double helix

• during DNA replication, each of the 2 strands are used as a template from which new strands are copied

semi conservative model?

• after replication, the new DNA is made up of one original polynucleotide strand, and a new complementary strand

• this explains why DNA replication is described as semi-conservative because one original strand is conserved

process of DNA replication: step 1?

• DNA helicase binds to the DNA and breaks the hydrogen bonds between the two strands

• the DNA helix unwinds and the 2 strands separate

process of replication: step 2?

• free floating nucleotides form hydrogen bonds with the complementary bases

• each strand is used as a template to produce complementary strands

process of replication: step 3?

• DNA polymerase forms phosphodiester bonds between the nucleotides

• 2 new DNA strands are synthesised

how are the carbon atoms of the pentose sugar in DNA numbered?

• the carbon atoms of the sugar molecule are number as 1’, 2’, 3’, 4’, 5’

how is each DNA strand different in its structure?

• one end is called the 3’ and the other end is called the 5’

• in a DNA helix, the strands run in opposite directions- they are antiparallel

what is the active site of the DNA polymerase complementary to (in terms of the ‘prime’ ends?

• it is only complementary to the 3’ end of the newly forming DNA strand, so the enzyme can only add nucleotides to the new strand at the 3’ end

how is the new strand made (in terms of primes)?

• it is made in a 5’ to 3’ direction and that DNA polymerase moves down the template strand in a 3’ to 5’ direction

• because the strands in DNA are antiparallel, the DNA polymerase working on one of the template strands moves in the opposite direction to the DNA polymerase working on the other template strand.

who were the 2 scientists that provided evidence for the semi-conservative model?

• in 1953, watson and crick thought that each strand acts as a template from which the new complementary strand was copied

• however, it was meselson and stahl who figured out that DNA replicates by the semi-conservation model

how did meselson and stahl conduct the experiment?

• they grew e.coli for several generations in one medium containing a ‘heavy’ isotope of nitrogen (15N), and another medium containing ‘light’ isotope of nitrogen (14N)

• during replication, the nitrogen gets included into the nitrogenous bases, and eventually into the DNA

meselson and stahl: isolating the DNA?

• after each of the first few generations, the cells were harvested and the DNA was isolated, then centrifuged

• the DNA from the heavy nitrogen settled lower down the centrifuge tube than the lighter nitrogen

meselson and stahl- conclusive evidence?

• the DNA harvested from cells grown for 2 generations in 14N formed 2 bands

  • one DNA band was between 15N and 14N

  • the other DNA band corresponded to the band of 14N DNA

• these results could only be explained if DNA replicates in a semi-conservative manner