Transcript dnarna-and-proteins

DNA,RNA, and
Proteins
• In the 1950’s, James Watson and
Francis Crick, built a model of DNA.
• Their model was inspired by the work of
Rosalind Franklin and Maurice Wilkins
who had made x-ray photographs of
DNA crystals.
Watson and Crick
Rosalind Franklin
• The model proposed by Watson and
Crick, not only explained the structure
of DNA but also showed how DNA was
able to replicate.
• You may remember that whenever cells
divide, the DNA must replicate(make a
copy of itself) During S phase
• DNA is made of two long chains
• Each chain is made of repeating units
called nucleotides.
• Each nucleotide is made of :
• 1. a sugar
• 2. a phosphate group
• 3. a nitrogenous base
• The sugar in DNA is deoxyribose sugar
• The phosphate group contains a
phosphorus atom and four oxygen
atoms.
• There are four kinds of nitrogenous
bases in DNA
• Two of the bases are called purines
because they have a double ring of
carbon and nitrogen atoms.
• The purines are adenine and guanine.
• Two of the bases have just one ring of
carbon and nitrogen atoms. They are
called pyrimidines.
• The pyrimidines are thymine and
cytosine.
• Watson and Crick determined that the
DNA molecule was a double helix in
which the sides of the ladder were
made of alternating sugars and
phosphates and the nitrogenous bases
formed the rungs.
• The rungs were always made of one
purine and one pyrimidine so that they
are all the same width.
Purines, Pyrimidines,and Nucleotides
• Erwin Chargaff had already observed
that the percentages of adenine and
thymine are always equal in 1949.
• This meant that adenine always joined
to thymine and cytosine always joined
to guanine.
• This is important because these base
pairing rules provide a way for DNA to
replicate.
Replication
• During S phase, enzymes cause the
DNA to unzip.
• Other enzymes help each half of the
DNA ladder to pick up nucleotides to
replace the missing half.
• Because of the base pairing rules , this
results in two identical molecules of
DNA.
DNA Replication
• Although DNA has all of the instructions
for building your proteins, DNA can’t
actually make them.
• The proteins are all made at the
ribosomes which are out in the
cytoplasm.
• DNA never leaves the nucleus.
• This means that DNA needs a
messenger.
• DNA makes a copy of its information in
the form of a different chemical called
RNA.
• RNA stands for ribonucleic acid
• RNA can leave the nucleus.
• RNA is also made of nucleotides.
• But:
• the sugar is ribose
• RNA does not contain thymine
and replaces it with uracil
• RNA is not a ladder. It is one
strand instead of a double strand
RNA
Transcription
• DNA makes RNA in a process called
transcription.
• Transcription means to copy something.
• Transcription is similar to the process of
replication.
• The DNA unzips and enzymes help the
DNA to pick up nucleotides to replace
the missing half
• The difference is that only one half of
the DNA is copied. This is important
because if it used both halves you
would get RNA with two different sets of
instructions.
• The other difference is the nucleotides
conatin ribose instead of deoxyribose
and uracil instead of thymine.
• There is more than one type of RNA.
• One type is called messenger RNA or
m-RNA.
• m-RNA carries the instructions for
building proteins to the ribosomes.
Translation
• Proteins are built by connecting amino
acids.
• There are 20 different amino acids.
• Each sequence of three nucleotides is
the code for one amino acid.
• Example: GCU is the code for alanine
but UGG is the code for tryptophan.
• A sequence of three nucleotides on
messenger RNA is called a codon.
• There is another kind of RNA called
transfer RNA or t=RNA.
•
Transfer RNA molecules pick up the
amino acids and take them to the mRNA.
Transfer RNA
• Each transfer RNA molecule can only
pick up one kind of amino acid.
• Each transfer RNA has an anticodon
which fits the codon on the m-RNA.
• This is how the amino acids get placed
in the correct order.
• Changing the genetic code into proteins
is called translation.
• Translation takes place at the
ribosomes
Translation
Genetic Code