Structure of DNA

1
Structure of DNA

• Base pairing
• Complementary strands
• 5' and 3' ends, antiparallel
• Leading and lagging strands
• Template and complementary strands (sense and
  antisense)

2
The two strands of DNA are antiparallel
3
Replication

• Replication origins and bubbles -single in
  prokaryotes, multiple in eukaryotes

4
Nucleoside triphosphates added
5
Replication

• DNA synthesis always starts with RNA primer (10
  bases) laid first, later replaced with DNA
• Leading strand is continuous
• Lagging strand is discontinuous, in Okasaki
  fragments (100-200 bases long)
• Ligase joins the fragments

6
Synthesis of leading and lagging strands during
DNA replication
7
Priming DNA synthesis with RNA
8
A summary of DNA replication
9
The end-replication problem
10
Question are only half the copies affected?
11
End-replication telomeres

• DNA polymerase cant finish the 5 ends
• DNA would get 10 bases shorter each time
  replicated
• Telomeres are expendable repetitive (TTAGGG)
  sequences at end of chromosomes
• Telomerase restores telomers, present in
  germ-line cells and cancerous cells

12
Telomeres and telomerase
13
Telomeres and telomerase Telomeres of mouse
chromosomes
14
RNA vs DNA

• RNA nucleotides have ribose not 2-deoxyribose

• uses one different base U (uracil) substituted
  for T (thymine)
• RNA is usually single stranded- not double (this
  doesn't change the information content why?)

15
Functions of RNA

• mRNA (messenger)- instructions protein
• tRNA (transfer) - carriers that match amino acids
  to codons during translation
• rRNA (ribosomal)- part of the ribosomes-
  ribosomes are RNA plus proteins
• snRNA's (small nuclear) - part of spliceosomes-
  that edit mRNA's
• siRNAs (small interfering) control gene
  expression.

16
Transcription

• Synthesis of RNA using DNA as a pattern
• Promoter sequence at beginning and terminator
  sequence at end of transcription unit (why not
  call it a gene?)
• RNA polymerases- (3 different ones in eukaryotes)
• Only template strand is transcribed

17
The stages of transcription initiation,
elongation, and termination (Layer 4)
18
The initiation of transcription at a eukaryotic
promoter
19
mRNA processing in eukaryotes

• Pre-mRNA (the raw transcript) is modified before
  leaving nucleus
• GTP cap and poly-A tail (a long string of
  adenosines) are added to 5 and 3 ends

20
mRNA processing, continued

• Non-coding introns are removed and the remaining
  exons are spliced together.

21
Roles of snRNPs and spliceosomes in mRNA splicing
22
Correspondence between exons and protein domains
23
Translation (protein synthesis)

• mRNA has the codons
• tRNAs deliver the amino acids and match
  anticodon to codon
• Ribosomes are where this happens. Catalyze
  formation of the peptide bonds.
• Aminoacyl tRNA synthetases reload the tRNAs with
  matching amino acids

24
Translation the basic concept
25
Representations of transfer RNAs (tRNA)
26
An aminoacyl-tRNA synthetase joins a specific
amino acid to a tRNA
27
(No Transcript)