Download Unit 9: DNA and RNA

DNA : Genetic Blueprints
The Double Helix

Watson and Crick
developed a model of
DNA called the Double
Helix

Two nucleotide chains that
wrap around each other to
form a double spiral or
double helix
Maurice Wilkins and Rosalind Franklin (1952) developed the Xray diffraction photographs of strands of DNA, which suggested
that the DNA resembled a tightly coiled double helix.
The Structure of DNA

DNA is a long molecule made up of units
called a nucleotides

Each nucleotide is made up of three basic parts
1. 5 carbon sugar – deoxyribose
2. Phosphate group
3. Four kinds of nitrogen bases
•
•
•
•
Adenine – A
Thymine – T
Cytosine – C
Guanine - G
The Structure of DNA
What are some things you observe about
the structure of DNA?
Chagraff’s Rule



A will only combine with T
C will only combine with G
Why?
Base-pairing




Adenine (A) and Guanine (G) are
categorized as purines, doubleringed.
Thymine (T) and Cytosine (C) are
categorized as pyrimidines,
single-rings.
Purines always match up with
pyrimidines; but, more
specifically, A always goes with T
and G always goes with C.
(Recall Chargaff’s Rule!)
Base pairs are held together by
Hydrogen bonds.
DNA Replication & the Cell Cycle
During what part of the cell cycle is DNA Replicated?
Replication
The Double Helix is “unzipped”
by DNA Helicase



Two nucleotide chains are seperated
Replication Fork formed
Base pairing occurs thanks to
DNA Polymerase



binds new nucleotides together
building a new DNA strand
Proof reads new strands
There were three alternative models of
DNA replication.
1. Conservative Model - two parental
strands reassociate after acting as
templates.
2. Semi-conservative Model - two
parental strands separate and each
serves as a template.
3. Dispersive Model - each strand of
daughter DNA contains a mixture of
parental and new DNA.
WHICH MODEL ACTUALLY OCCURS
IN DNA RPELICATION?
DNA REPLICATION IS:
SEMICONSERVATIVE
DNA Replication: A Closer Look
Enzymes, called DNA helicase, bind to origins of replication on the
double helix. DNA helicases break the H bonds holding
complementary strands together. Once the two strands are separated,
additional proteins attach to each strand, holding them apart. The
areas where the double helix separates are called replication forks.
After DNA helicases bind to origins of replication and replication forks
have been established, new complementary strands must be formed….
At the replication fork,
enzymes known as DNA
polymerases move along
each of the parent DNA
strands (in a 5’ to 3’
direction) adding
complementary
nucleotides according to
base-pairing rules.
DNA ligase attaches and
follows along sealing up
“gaps” between Okazaki
fragments.
What happens if the wrong
nucleotide is placed in the
new strand?
Can mistakes be corrected?
Mistakes in replication occur all the
time, however, your cell has adapted a
procedure to “proofread” the new
DNA.
If a mistake is made in the new
strand of DNA:
1. a nuclease enzyme cuts the
damaged DNA strand at two points
and the damaged section in removed.
2. DNA polymerase comes back and
fills in the missing nucleotides.
3. Then DNA ligase seals the free end
of the new DNA to the old DNA,
making the strand complete.
Proofreading reduces errors in DNA
replication to about one error per 1
billion nucleotides.
The Central Dogma
Gene to Proteins: Transcription & Translation
RNA and Protein Synthesis
RNA

The nucleic acid, RNA is responsible for the
movement of genetic information from the
DNA in the nucleus to the ribosome where
proteins are made.
DNA is like the boss who cannot leave the
office, RNA is the worker who does
whatever the boss tells them


•
RNA travels outside the nucleus carrying info
from DNA
Structure of RNA

Nucleotides
5 carbon sugar- RIBOSE
 Phosphate group
 Nitrogen Bases (A, U, C, & G)

Single Stranded
 Uracil – replaces Thymine

Types of RNA



Messenger RNA (mRNA) consists of nucleotides in a
single strand - carries info for DNA from the nucleus
to the ribosome
Transfer RNA (tRNA) binds to ribosome and transfers
amino acids to make proteins
Ribosomal RNA (rRNA) many nucleotides make up
the ribosomes
Gene to Proteins

Protein Synthesis has TWO major steps:
Transcription
 Translation

Transcription
Adenine (DNA and RNA)
Cystosine (DNA and RNA)
Guanine(DNA and RNA)
Thymine (DNA only)
Uracil (RNA only)
RNA
polymerase
DNA
RNA
Transcription
Process where genetic information is copied from
DNA and RNA

Step 1: DNA is Separated



Helicase – separates the DNA strands by breaking the
hydrogen bonds
RNA Polymerase – enzyme that builds RNA from DNA
Promoters – tells DNA where to start making new RNA
•
•
In eukaryotes promoters mark the beginning of a gene.
In prokaryotes promoters mark the beginning of many genes.
Transcription
Step 2: RNA Polymerase


Enzyme – attach new RNA molecule to the DNA molecule
Single Side – occurs on only one side of DNA
Step 3: Termination Signal


Transcription continues until it reaches a stop
All 3 types of RNA are made this Way
Transcription is DNA making RNA.
RNA editing (spell check)
RNA molecules have sections called introns which are edited out
before they become functional



Introns are “junk” or “intervening sequences” We still don’t fully understand
these segments.
Exons – remaining pieces of RNA that actually code for proteins
The Genetic Code
DNA gives its info to mRNA
 The sequence of nucleotides in RNA is
translated into amino acids which make
up proteins
 codon – a combination of 3 nucelotides
that codes for a specific amino acid


Ex. AAA- Lysine
Genetic Code
Genetic Code

The production of proteins is called
protein synthesis
polypeptides – proteins are made by
connecting amino acids by a peptide bond
 20 possible amino acids

• 64 possible codons- there are not 64 actual amino
acids produced
• Multiple codons can code for the same amino acid
Step 2. Translation
During translation, the cell uses
information from mRNA to produce
proteins.



The cell uses all three forms of RNA in this
process.
mRNA – is made in nucleus and attaches
to the ribosome
Translation - 1
Translation -2