7.1 dna structure and replication

7.1 DNA Structure and Replication

GuidanceDetails of DNA replication between prokaryotes and eukaryotes differ. Only the prokaryotic system is expected.The protein enzymes involved in DNA replication should include helicase, DNA gyrase, single strand binding proteins, DNA primase, and DNA polymerases I and IIIThe regions of DNA that do not code for proteins should be limited to regulators of gene expression, introns, telomeres, and genes for tRNAs.

Understandings• Nucleosomes help to supercoil the DNA• DNA structure suggested a mechanism for

DNA replication• DNA polymersases can only add nucleotides to

the 3’ end of a primer• DNA replication is continuous on the leading

strand and discontinuous on the lagging strand• DNA replication is carried out by a complex

system of enzymes• Some regions of DNA do not code for proteins

but have other important functions

Applications/Skills• A: Rosalind Franklin and Maurice Wilkins

investigation of DNA by x-ray diffraction• A: Use of nucleotides containing

dideoxyribonucleic acids to stop DNA replication in preparation of samples for base sequencing

• A: Tandem repeats are used in DNA profiling• S: Analysis of results of the Hershey and Chase

experiment to provide evidence that DNA is the genetic material

• S: Utilization of molecular visualization software to analyze the association between protein and DNA within a nucleosome

DNA is the Genetic Material• Alfred Hershey and Martha

Chase showed DNA contains the genetic information, not protein• Two cultures of bacteriophages:

one with radioactive sulfur (component of protein) and one with radioactive phosphorous (component of DNA)

http://www.hyglos.de/typo3temp/pics/f248049cfa.jpg

• The bacteriophages were then used to infect E. coli• One culture of E. coli contained

radioactive P within the cell wall; the other contained no radioactive S within the cell wall• This showed that DNA was

transferred from the bacteriophage to the bacteria and must be the substance containing genetic information

http://www.hyglos.de/typo3temp/pics/f248049cfa.jpg

DNA Structure• Nucleotides are attached by

phophodiester bonds • Condensation reaction• New nucleotides are always attached

to the 3’ end of the previous nucleotide

http://usmle.biochemistryformedics.com/wp-content/uploads/3-5phosphodiester-linkage.jpg

Structure continued• Purines are double ringed structures

(Guanine and Adenine)• Pyrimidines are single ringed

structures (Cytosine and Thymine) • A-T: two hydrogen bonds• C-G: three hydrogen bonds

DNA Packaging• DNA is organized into nucleosomes• 8 histone proteins (positively

charged)• DNA (negatively charged) wraps

around twice• H1 histone to help maintain structure

of nucleosome

• Further wrapping of DNA into supercoiled chromosomes

http://www.mit.edu/~kardar/teaching/projects/dna_packing_website/histone_dna.jpg

https://beyondthedish.files.wordpress.com/2013/07/chromatin1.jpg

DNA Sequences• Highly repetitive sequences• About 45% of total DNA• 5-300 bp’s• Up to 100 000 replicates• Sometimes called jumping genes due to the fact that they can move around

the chromosome that they are a part of• No coding functions

DNA Sequences• Protein-coding genes• Less than 2% of DNA• Single copy genes with coding function• Contains code for producing a protein• Interspersed among noncoding fragments (introns)

DNA Sequences• Structural DNA• About 20% of DNA• No coding function• Around the centromere and telomeres

Summary of Replication

Helicase binds at the origin and breaks H bonds between nitrogenous bases

Replication forks

1

2

Primase attaches at the fork and produces an RNA primer

3

DNA polymerase III adds nucleotides in the 5’ to 3’ direction4

DNA polymerase I removes the RNA primer and replaces it with DNA nucleotides

5

Assembled continuously toward the fork

Assembled discontinuously toward the fork (Okazaki fragments)

DNA ligase seals Okazaki fragments together

PCR