April 9th - Wednesday

Corrections: (4/13/08) -- Found that the last line stated 3 nt = 1 amino acid = 43 = 64.

It should've said 4³, NOT 43. Sorry for any inconvenience - (I'm type fast, but its hard to format charts, subscripts, superscripts, etc.)

it has been corrected

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4-9-08

 

NOTE: We made some additional notes on slides!

(Figure LE16-16 Explained.)

Figure LE 16 -16 Explained (& revised)
Overall replication moving to the left. (5' to 3') as per big arrows. The green blob thing in the backround of the picture is the Helicase enzyme The grey little circles next to the helicase are single strand binding proteins   To make DNA you must have a free end, and DNA can only be added to an existing piece of DNA or Rna, so the red blocks are the primer.

 

Movie: A video clip on the synthesis of DNA - on CD called "A closer look DNA syn."

 

Telomeres - ends of chromosomes ("telo -" end ; "-mere"  body/thing)

Fig 16.18 - LOOK HERE to understand THIS:

 

We said before that all RNA needs to be replaced with DNA.

But we have this RNA primer, which is needed by the DNA Polymerase to function

Remember? The polymerase won't work because there's no existing "stuff" to work off-of. It can build upon existing lines, but it can't start them.

So, when we lose our RNA polymer, we also lose about 10 nucleotides on average.

This doesn't sound like a lot, but because we replicate so often, it adds up big-time.

 

ON FIGURE 16.18

RNA primer -the red thing at the end- can't be filled-in with DNA after its lost, but we can't put anything in there because DNA polymerase has to have a primer.

 

.cam notes:

Problem because DNA polymerase has to have a primer. So the lagging strand at the end of every chromosome will be shorter after every DNA replication.

 

The Solution? Telomerase!

 

Telomerase - enzyme that adds DNA to the ends of chromosomes.

• Has an RNA sequence that is part of the enzyme.  --(One of the most unique enzymes in all Biology because of this)
• Uses that sequence as a template to add many copies of the sequence to the ends of the chromosomes.

• Sequence - ~6 bp (base pairs) long   -- Telomere sequence.

  (so it doesn't code for anything)

     When does this happen?

Happens in the S-phase right before meiosis.

every division after that results in shorter telomeres. They believe it has something to do with aging.

Cancer Cells - they make their own telomerase - so they keep rejuvenating their chromosome ends to keep replicating & makes their cells immortal.

(some cells in labs are still dividing since the 60's.)

 

Accuracy of the DNA synthesis:

1.) Proof reading: DNA polymerase checks the DNA as it synthesizes it. ( it's like a backspace, erase, and replace function)

2.) Mismatch repair enzymes: recognize errors in the DNA sequence & repair them.

• replication errors DNA polymerase missed, or
• induced mutations (chemicals, uv radiation, etc) - a biggie in cancers. (Many cancers running in families are due to a defect in this error check.)

 

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CHAPTER 17: From gene --> proteins

Central Dogma:   DNA ---> RNA ----> Protein

Chromosome DNA:  Gene is a DNA sequence that codes for one protein.

1 gene ---> 1 protein.

 

In general:

Average gene ~ 1200 bp

Average protein ~400 amino acids

 

        DNA --------------->RNA   -----------------> Protein

              "Transcription"             "Translation"

 

Bacteria:   (Have no nucleus);  transcription and translation occur at the same time.

 

Eukaryotes: (have nucleus)

• Transcription occurs in the nucleus
• RNA Processing occurs in the nucleus

Then the mRNA (message RNA) moves out of the nucleus through the nuclear pores into the cytosol.

In the cytosol, there are ribosomes - which is composed of  rRNA + Protein (rRNA = ribosomal RNA)

• Translation occurs in the cytosol - careful, this one's different!
• Eukaryotes have more control over gene expression by this separation of events in the cell.

 

SO THE BIG OVERVIEW- Principle of transcription:

DNA---------> mRNA

A complementary messager RNA molecule is made from a DNA template in the nucleus.

 

Review:             RNA   vs. DNA

RNA	DNA
Single stranded	Double stranded
Ribose	Deoxyribose (sugars)
Uracil	Thymine

BOTH: Use Adenine, Guanine, and Cytosine.

 

Enzyme that synthesizes RNA ---> RNA polymerase.

Like DNA, RNA is synthesized in the 5' to 3' direction only.

 

mRNA -------> Protein

mRNA sequence is tranlated to a specific amino acid sequence in the cytosol by ribosomes.

 

The Genetic Code:

• 4 different nucleotides ( A,U,C, G)
• 20 different amino acids used to make proteins.

How do you read the mRNA sequence?
  1 nt (nucleotide) equals 1 amino acid  4¹ = codes only 4 different amino acids (but we need 20, so no good.)

  2 nt (nucleotide) equals 1 amino acid = 4² = 16 different amino acids -(but we need 20, so it's still no good.)

3 nt = 1 amino acid = 4³ = 64 different amino acid possibilities --  THAT'S IT!

a tripletcode of 3 nucleotides --> 1 amino acid ---> is called a Codon.