Upload
esther-cory-snow
View
214
Download
0
Tags:
Embed Size (px)
Citation preview
What do you know about DNA?• Building blocks are called?
– nucleotides
• The shape is ?– Double helix
• The three primary components of a nucleotide are?– Sugar, phosphate, nitrogen base
• Where are each of these found?– The sugar and phosphate are in the backbone; the nitrogen base is on the rungs between DNA strands
• The sugar is?– deoxyribose
DNA Primary Structure Summary• Backbone (sugar+phophate with phosphate ester bonds and base side chains)
• Each strand of the double helix is oriented in the opposite direction
• What is this arrangement called?
• “Antiparallel “
Figure 10.5B
5 end 3 end
3 end 5 end
P
P
P
PP
P
P
P
What do you remember?
• What does semi conservative replication mean?
– The resulting DNA includes one original strand and one new strand
– One original strand is “conserved” or saved.
Untwisting and replication of DNA
• each strand is a template for a new strand
Figure 10.4B
helicase
DNA polymerase
What is the job of the helicase enzyme?– To unzip the DNA
What is the job of the DNA polymerases?
– To build new DNA strands by attaching nucleotides to the parent strand
What is the job of ligase?– To attach fragments of DNA together
• DNA polymerase works in only one direction
5 end
P
P
Parental DNA
Figure 10.5C
DNA polymerasemolecule
53
35
35
Daughter strandsynthesizedcontinuously
Daughter strandsynthesizedin pieces
DNA ligase
Overall direction of replication
53
• Telomere sequences are lost with each replication.
• Cancer, aging
telomeres
So what is a telomere anyway?
• A telomere is a repeating DNA sequence (for example, TTAGGG) at the end of the body's chromosomes.
• The telomere can reach a length of 15,000 base pairs. • Telomeres function by preventing chromosomes from losing base pair sequences at their ends. They also stop chromosomes from fusing to each other.
• Each time a cell divides, some of the telomere is lost (usually 25-200 base pairs per division).
• When the telomere becomes too short, the chromosome reaches a "critical length" and can no longer replicate.
• This means that a cell becomes "old" and dies by a process called apoptosis.
So what do telomeres have to do with
Cancer?• Its all about the enzyme, telomerase.• Telomerase causes the continued addition of DNA ends (telomeres) so cells with active production of telomerase continue to grow and divide. Body cells no longer produce telemerase.
• What kind of cells keep growing and dividing?
• Fetal cells, tumor cells including cancer cells
• DNA replication begins at many specific sites
How can entire chromosomes be replicated during S phase of the cell cycle?
Figure 10.5A
Parental strandOrigin of replication
Bubble
Two daughter DNA molecules
Daughter strand
What are these sites called?Replication Bubbles
• In transcription, DNA helix unzips
– RNA nucleotides line up along one strand of DNA, following the base-pairing rules
– single-stranded messenger RNA peels away and DNA strands rejoin
– What are the 3 steps of transcription?
RNA polymerase
DNA of gene
PromoterDNA Terminator
DNAInitiation
Elongation
Termination
Area shownin Figure 10.9A
GrowingRNA
RNApolymerase
Completed RNA
Figure 10.9B
Eukaryotic RNA is processed before leaving the nucleus
• Noncoding segments, introns, are spliced out
• A cap and a tail are added to the ends
Figure 10.10
DNA
RNAtranscriptwith capand tail
mRNA
Exon Intron IntronExon Exon
TranscriptionAddition of cap and tail
Introns removed
Exons spliced together
Coding sequence
NUCLEUS
CYTOPLASM
Tail
Cap
RNA vs. DNA
• RNA contains Uracil, not Thymine
• Sugar is Ribose• Usually single stranded
Nitrogenous base(A, G, C, or U)
Sugar(ribose)
Phosphategroup
Types of RNA• tRNA - Transfer RNA carries amino acid and read codons on m-RNA through its
own anticodons.
Types of RNA• tRNA - Transfer RNA carries amino acid and read codons on m-RNA through its
own anticodons.
Check
• The following section of DNA is used to build a strand of mRNA
• 3’ – GAA- CCC- TTT- 5’• What is the corresponding mRNA sequesnce?
• What are the anticodons on the tRNA in the next step?Modified from www.elmhurst.edu/~chm/onlcourse/CHM103/Rx22DNA.ppt
• Summary of transcription and translation
Figure 10.15
1Stage mRNA istranscribed from aDNA template.
Anticodon
DNA
mRNARNApolymerase
TRANSLATION
Enzyme
Amino acid
tRNA
InitiatortRNA
Largeribosomalsubunit
Smallribosomalsubunit
mRNA
Start Codon
2Stage Each amino acid attaches to its proper tRNA with the help of a specific enzyme and ATP.
3Stage Initiation of polypeptide synthesis
The mRNA, the first tRNA, and the ribosomal subunits come together.
TRANSCRIPTION
Figure 10.15 (continued)
4Stage ElongationGrowingpolypeptide
Codons
5Stage Termination
mRNA
Newpeptidebondforming
Stop Codon
The ribosome recognizes a stop codon. The poly-peptide is terminated and released.
A succession of tRNAs add their amino acids to the polypeptide chain as the mRNA is moved through the ribosome, one codon at a time.
Polypeptide