Biology 12
Hand in Vitamin C assignment
Finish GATTACA movie & journal page
Reminder - Protein synthesis/Mutation quiz next class
Learning Outcomes for quiz:
- Identify roles of DNA, mRNA, tRNA and ribosomes in transciption and translation
- Determine a sequence of amino acids from DNA using a mRNA codon table
- Use examples to explain how mutations in DNA change the sequence of amino acids and as a result may lead to genetic disorders
- Define and give examples of mutagens
DNA Replication Update:
Here is some more information on DNA Replication & a 'interesting' rap that clearly explains the whole process...
Rap Video: https://www.youtube.com/watch?v=1L8Xb6j7A4w
Rap Video: https://www.youtube.com/watch?v=1L8Xb6j7A4w
DNA Replication is Semi-Conservative
DNA replication of one helix of DNA results in two identical helices. If
the original DNA helix is called the "parental" DNA, the two
resulting helices can be called "daughter" helices. Each of these two
daughter helices is a nearly exact copy of the parental helix (it is not 100%
the same due to mutations). DNA creates "daughters" by using the
parental strands of DNA as a template or guide. Each newly synthesized strand
of DNA (daughter strand) is made by the addition of a nucleotide that is
complementary to the parent strand of DNA. In this way, DNA replication is
semi-conservative, meaning that one parent strand is always passed on to the
daughter helix of DNA.
The first step in DNA replication is the separation of the two DNA
strands that make up the helix that is to be copied. DNA Helicase untwists the
helix at locations called replication origins. The replication origin forms a Y
shape, and is called a replication fork. The replication fork moves down the
DNA strand, usually from an internal location to the strand's end. The result
is that every replication fork has a twin replication fork, moving in the
opposite direction from that same internal location to the strand's opposite
end.
When the two parent strands of DNA are separated to begin replication,
one strand is oriented in the 5' to 3' direction while the other strand is
oriented in the 3' to 5' direction. DNA replication, however, is inflexible:
the enzyme that carries out the replication, DNA polymerase, only functions in
the 5' to 3' direction. This characteristic of DNA polymerase means that the
daughter strands synthesize through different methods, one adding nucleotides
one by one in the direction of the replication fork, the other able to add
nucleotides only in chunks. The first strand, which replicates nucleotides one
by one is called the leading strand; the other strand, which replicates in
chunks, is called the lagging strand.
The Leading Strand
Since DNA replication moves along the parent strand in the 5' to 3'
direction, replication can occur very easily on the leading strand. As seen in
, the nucleotides are added in the 5' to 3' direction. Triggered by RNA
primase, which adds the first nucleotide to the nascent chain, the DNA
polymerase simply sits near the replication fork, moving as the fork does,
adding nucleotides one after the other, preserving the proper anti-parallel
orientation.
The Lagging Strand
The lagging strand replicates in small segments, called Okazaki
fragments. These fragments are stretches of 100 to 200 nucleotides in humans that
are synthesized in the 5' to 3' direction away from the replication fork. Yet
while each individual segment is replicated away from the replication fork,
each subsequent Okazaki fragment is replicated more closely to the receding
replication fork than the fragment before. The lagging strand must wait for a
patch of the parent helix to open up a short distance in front of the newly
synthesized strand before it can begin its synthesis back to the end of the
daughter strand. This "lag" time does not occur in the leading strand
because it synthesizes the new strand by following right behind as the helix
unwinds at the replication fork.
These fragments are then stitched
together by DNA ligase, creating a continuous strand.