DNA

DNAThe Molecule of Life

Specific ObjectivesIllustrate the structure of RNA and DNA Draw a nucleotideRecognize the structural formulae of nucleotides, ribose, deoxyribose, pyrimidines and purinesExplain the importance of hydrogen bonding and base pairing in DNA replication

What is DNA?DeoxyriboNucleic Acid

The backbone of DNA is based on a repeated pattern of a sugar group and a phosphate group. The full name of DNA, deoxyribonucleic acid, gives you the name of the sugar present - deoxyribose.

James Watson and Francis CrickModel of DNADouble stranded structureBases inside3Deoxyribose

What is DNA Made of?Deoxyribose sugar Phosphate Base (nitrogenous base):PurineA, GPyrimidineT, C

What are the Structures of the bases?PurinesAdenine

Guanine

What are the structures of the bases?PyrimidinesThymine

Cytosine

Base pairing: how it worksHydrogen Bonding between bases:A-T Bonding2 hydrogen bonds (double bond)G-C Bonding3 hydrogen bonds (triple bond)Donor and AcceptorA View of base pairingA:T

A View of Base PairingG:C

What is important about base pairs?Can predict sequence of one strand based on the sequence of the other.Replication and TranscriptionRepair of damaged DNALinking it all together: Nucleotide

A, G, C or TWhat makes DNA Different from RNA?Forms sugar Phosphate BackboneSpecific ObjectivesExplain DNA as having an anti-parallel structureExplain the significance of the 5 and 3 end of DNA in DNA replicationList the proteins /enzymes involved in DNA replicationExplain how DNA replication occurs in a semi-conservative mannerExplain the process of DNA replicationJoining the nucleotides into a DNA strandA DNA strand is simply a string of nucleotides joined together. The phosphate group on one nucleotide links to the 3' carbon atom on the sugar of another one. In the process, a molecule of water is lost (condensation reaction).

Joining the nucleotides into a DNA strandA DNA strand is simply a string of nucleotides joined together. The phosphate group on one nucleotide links to the 3' carbon atom on the sugar of another one. In the process, a molecule of water is lost -condensation reaction.

Joining the nucleotides into a DNA strand

Anti-parallel structure

Joining the nucleotides into a DNA strandWhat matters in DNA is the sequence the four bases take up in the chainUse a simplified diagram in building up the chain

The individual bases are identified by the first letters of the base names. (A = adenine, etc). There are two different sizes of base. Adenine and guanine are bigger because they both have two rings. Cytosine and thymine only have one ring each.

Joining the nucleotides into a DNA strand

Joining the two DNA chains togetherA adenine on one chain is always paired with a thymine on the second chain. And a guanine on one chain is always paired with a cytosine on the other one.Smaller base is always paired with a bigger one. WHY ?

Joining the two DNA chains togetherThe pairing has to beexactlyadenine (A) pairs with thymine (T);guanine (G) pairs with cytosine (C).That is because these particular pairs fit exactly to form very effective hydrogen bonds with each other. It is these hydrogen bonds which hold the two chains together.

Joining the two DNA chains togetherThe pairing has to beexactlyadenine (A) pairs with thymine (T);guanine (G) pairs with cytosine (C).

Joining the two DNA chains together

5 3 notation will become important when we discuss genetic codeReplication:Why?When cells replicate, each new cell needs its own copy of DNA.Where?Nucleus in Eukaryotes. Cytosol in ProkaryotesWhen?S phase of cell cycleWhat?Many proteins: major is DNA PolymeraseHow?Three theories for DNA replicationSemi-conservative ConservativeDispersiveThree theories for DNA replication

Three theories of DNA replication

Semi-conservative replicationA very simple look at the processFirst two individual strands in the DNA double helix start to unzip under the influence of the enzyme helicase.The diagram shows this happeningin the middle of the DNA double helix- you mustn't assume that the top of the diagram is the end of the chain. It isn't. Further up the double helix, the two strands will still be joined together.In fact, this is happening lots of times along the very long DNA molecule. Lengths of chain become separated to form what are known as "bubbles".

Semi-conservative replicationA very simple look at the processFirst two individual strands in the DNA double helix start to unzip under the influence of the enzyme helicase.The diagram shows this happeningin the middle of the DNA double helix- you mustn't assume that the top of the diagram is the end of the chain. It isn't. Further up the double helix, the two strands will still be joined together.In fact, this is happening lots of times along the very long DNA molecule. Lengths of chain become separated to form what are known as "bubbles".

Semi-conservative replicationA very simple look at the processNow suppose have a source of nucleotides - phosphate joined to deoxyribose joined to a base, including all the four sorts of bases needed for DNA.The next diagram shows what would happen if a nucleotide containing guanine (G) and one containing cytosine (C) were attracted to the top two bases on the left-hand strand of the unzipped DNA - and then joined together.

Semi-conservative replicationA very simple look at the processNow suppose have a source of nucleotides - phosphate joined to deoxyribose joined to a base, including all the four sorts of bases needed for DNA.The next diagram shows what would happen if a nucleotide containing guanine (G) and one containing cytosine (C) were attracted to the top two bases on the left-hand strand of the unzipped DNA - and then joined together.

This takes place under the influence of enzymes including DNA polymerase I and IIISemi-conservative replicationA very simple look at the processCompare the double strands that formed on the left- and right-hand sides. They are exactly the same . . . and if we continue this process, they would continue to be the same.And if you compare the patterns of bases in the new DNA being formed with what was in the original DNA before it started to unzip, everything is the same. This is inevitable because of the way the bases pair together.

Semi-conservative replicationWhat does semi conservative mean ?The original DNA is shown all in blue. The red strands in the daughter DNA are the ones which have been built on the original blue strands during the replication process.You can see that each of the daughter molecules is made of half of the original DNA plus a new strand.

That's all "semi-conservative replication" means. Half of the original DNA is conserved (kept) in each of the daughter molecules.

DNA replication

DNA replication

RNAThe sequence of bases in DNA carries thegenetic code. Scattered along the DNA molecule are particularly important sequences of bases known asgenes. Each gene is a coded description for making a particular protein.RNAGetting from the code in DNA to the final protein is a very complicated process.The code is first transcribed ("copied", although with one important difference - see later) to messenger RNA. That then travels out of the nucleus of the cell (where the DNA is found) into the cytoplasm of the cell. The cytoplasm contains essentially everything else in the cell apart from the nucleus. Here the code is read and the protein is synthesised with the help of two other forms of RNA - ribosomal RNA and transfer RNA. RNAHow does messenger RNA differ from DNA?1. LengthRNA is much shorter than DNA. DNA contains the code for making lots and lots of different proteins. Messenger RNA contains the information to make just one single polypeptide chain - in other words for just one protein, or even just a part of a protein if it is made up of more than one polypeptide chain.

RNAHow does messenger RNA differ from DNA?2. Overall structureDNA has two strands arranged in a double helix. RNA consists of a single strand.3. The sugar present in the backbone of the chainDNA (deoxyribonucleic acid) has a backbone of alternating deoxyribose and phosphate groups. In RNA (ribonucleic acid), the sugar ribose replaces deoxyribose.

RNAHow does messenger RNA differ from DNA?

4. RNA uses the base uracil (U) rather than thymine (T)The structure of uracil is very similar to that of thymine.The only difference between the two molecules is the presence or absence of the CH3group.

ReplicationHow?53 directionalityStarts with RNA primerLeading StrandLagging StrandOkasaki FragmentsSequence determined by basepairingDNA Replication Animation

TranscriptionDNA RNAWhat is the difference between DNA and RNA?Ribose SugarUracil not thymine

TranscriptionWhere?Nucleus in EukaryotesCytosol in ProkaryotesWhat? RNA Polymerase plus some minor proteinsWhen? When RNA is neededWhy?RNAs serve many important functions in cellsHow?