Download 12-2 Notes

12-2 Notes
DNA and RNA
Structure of DNA
DNA is a long molecule made up of
units called nucleotides
Each nucleotide has three
components
1. a 5 carbon sugar called deoxyribose
2. a phosphate group
3. a nitrogenous base
Structure of DNA
 There are four nitrogenous bases that
belong in two categories
 Purines – have two rings in their structure
 Adenine
 Guanine
 Pyrimidines – have one ring in their structure
 Cytosine
 Thymine
Base Pairing
 DNA is a double helix (two strands bonded
together)
 The nitrogenous bases bind the two
strands
 The bases pair in very specific ways
 Adenine always binds with Thymine
 Cytosine always binds with Guanine
Nucleotide
Hydrogen
bonds
Sugar-phosphate
backbone
Key
Adenine (A)
Thymine (T)
Cytosine (C)
Guanine (G)
DNA and Chromosomes
 DNA molecules are very long
 Ex. E. coli’s DNA is 4,639,221 base pairs long
(about 1.6 mm)
 This DNA must be folded to 1/1000th of its
length just to fit inside the bacteria
Chromosome Structure
Eukaryotic chromosomes contain
both DNA and protein
Chromatin is DNA coiled tightly
around proteins called histones
Together these form a structure
called a nucleosome
Nucleosomes are shortened by a series
of loops and coils
Nucleosome
Chromosome
DNA
double
helix
Coils
Supercoils
Histones
DNA Replication
 Before a cell divides, it copies its DNA in a
process called replication
 It can occur at hundreds of places along a
chromosome
 It will proceed in both directions until
replication is complete
 Each site where replication is started is called a
replication fork
New strand
DNA
polymerase
Original
strand
DNA
polymerase
Growth
Replication
fork
Replication
fork
New strand
Original
strand
Nitrogenous
bases
Duplicating DNA
 During replication, the DNA molecules
separates into two strands, then produces
two new complementary strands following
the rules of base pairing
 Each strand of the double helix of DNA
serves as a template for the new strand
How Replication Occurs
 Various enzymes aid in replication
 They “unzip” DNA into two strands
 DNA polymerase is an enzyme that joins
individual nucleotides to produce a new
DNA molecule
 DNA polymerase also “proofreads” each
new DNA strand to minimize mistakes
12-3 Notes
RNA and Protein Synthesis
The Structure of RNA
 RNA consists of a long chain of
nucleotides.
 The sugar in RNA is ribose.
 RNA is single-stranded and contains uracil
in place of thymine.
Types of RNA
 RNA’s main function is protein synthesis.
 The assembly of amino acids into proteins
is controlled by RNA.
 There are three main types of RNA:
Types of RNA
 1.) Messenger RNA (mRNA): carries the
instructions for assembling amino acids into
proteins.
 They serve as “messengers” from DNA to the rest of
the cell.
 2.)Ribosomal RNA (rRNA): where proteins are
assembled on ribosomes.
 3.)Transfer RNA (tRNA): transfers each amino
acid to the ribosome as it is specified by
coded messages in mRNA.
Transcription
 During transcription, RNA polymerase
binds to DNA and separates the DNA
strands.
 RNA polymerase then uses one strand of
DNA as a template to attach the
nucleotides.
 Signals called promoters indicates where
to bind to and when to stop making the
copy.
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
The Genetic Code
 Proteins are made by joining amino acids
into long chains called polypeptides.
 Each polypeptide contains a combination
of any or all of the 20 different amino
acids.
 The “language” of mRNA instructions is
called the genetic code.
The Genetic Code
 The genetic code is read three letters at a
time.
 Each three letter “word” in mRNA is called
a codon.
 It consists of three consecutive
nucleotides that specify for a single amino
acid.
 For what amino acid does the codon UGC code?
 Cysteine
Translation
 The decoding of an mRNA message into a
polypeptide chain is known as translation.
 Translation takes place on ribosomes.
 During translation, or protein synthesis,
the cell uses information from messenger
RNA to produce proteins.
Genes and Proteins
 Proteins are enzymes that regulate
chemical reactions.
 They are each designed to build or
operate a component of a living cell.
 Genes for certain proteins can regulate
the rate and pattern of growth.
 They also control an organisms size and
shape.