Powers Unit 5 Notes Final

The Molecule of Life

H.B.4

•The student will demonstrate an
understanding of the specific
mechanisms by which characteristics
or traits are transferred from one
generation to the next via genes.

DNA

DNA is the molecule of life. All living things contain

DNA.

DNA is found in the nucleus of cells.

DNA contains genetic codes that determine physical

features.

DNA

DNA stands for deoxyribonucleic acid. It may sound

gibberish, but the name actually tells us two things:

- DNA contains deoxyribose (a 5-carbon sugar)

- DNA is a nucleic acid (molecule made up of nucleotides)

Nucleotide 🡪

Nucleotides consist of a sugar molecule attached to a

nitrogen base and a phosphate group.

Sugar

Phosphate

Nitrogen Base

Nucleotides have 3 parts:
1)

Sugar

2)

Nitrogen Base

3)

Phosphate

THE NITROGEN BASES

There are 4 possible different nitrogen bases:

-adenine

-guanine

-cytosine

-thymine

These 4 different bases

allow for genetic

diversity

BASE-PAIRING RULES

DNA is a double stranded molecule – the two

strands are connected by the nitrogen bases.

Adenine can only pair with thymine (and vice versa).

Guanine can only pair with cytosine (and vice versa).

C

G

T

A

PURINES VS. PYRIMIDINES

The molecular structure of the 4 bases fall under two categories:

1)

Purines- double ring structures

- adenine and guanine are purines

2) Pyrimidines- single ring structures

- thymine and cytosine are pyrimidines

Purines always bond with pyrimidines (as per the base pairing

rules).

Think OPPOSITES- The BIGGER word is the

smaller molecule; the smaller word is the

BIGGER molecule.

CHARGAFF’S RULE

Chargaff's rules states that DNA from any cell should
have a 1:1 ratio of pyrimidine and purine bases (as per
the base pairing rules).

•In other words, the amount of guanine is equal to
cytosine and the amount of adenine is equal to
thymine.

Erwin Chargaff developed
Chargaff’s rules through careful
experimentation.

His discoveries helpedWatson
and Crick develop their model of
the double helix.

CHARGAFF’S RULE

Example: If a DNA molecule contains 28% cytosine,
we can figure out how much guanine, thymine, and
adenine are present in the molecule.

Cytosine

28%
According to

Chargaff’s rule,

how much
guanine is
present?

Guanine

28%

How much of the

DNA have we

accounted for so

far?

28% + 28% = 56%

This tells us that 44% of

the molecule must be

made up of adenine and

thymine.

Adenine

22%

Thymine

22%

WATSON AND CRICK

The shape of DNA is very complex.

In 1953, James Watson and Francis Crick determined the shape of

DNA based on X-ray diffraction.

THE STRUCTURE OF DNA:

DISCOVERY

Though Watson and Crick tend to get most of the credit
for discovering the structure of DNA, their discovery was
more of a puzzle completion.

• The work and information of other scientists was used

to determine the structure of DNA, namely Rosalind
Franklin.

Multiple Choice

Who is responsible for producing this image?

• Francis Crick

• James Watson

• Rosalind Franklin

• Watson and Crick

Multiple Choice

How did Rosalind Franklin's photo 51 affect the work of Watson and Crick?

• It was used to determine the physical structure of DNA

• It was used to identify the four bases that make up DNA

• It was used to determine the theory of independent assortment

• It was used to show DNA was the molecule of inheritance

Multiple Choice

What did Erwin Chargaff conclude from his experiments?

• Nitrogen bases are held together by peptide bonds

• A and T, C and G always occur in equal amounts

• Proteins are the carriers of genetic information

• C and G rarely occur in eukaryotic organisms

If DNA is straightened
out and flattened, it
looks like a ladder.

The Double Helix

The sides of the ladder are

composed of sugar
(deoxyribose) molecules
and phosphates. This is
called the
“sugar-phosphate
backbone”.

The nitrogen bases make

up the rungs (steps) of the
ladder.

The two strands of DNA are
connected to each other at the bases.

The bases bond together using
hydrogen bonds.

Adenine and thymine have two
hydrogen bonds.

Guanine and cytosine have three
hydrogen bonds.

Hydrogen bonds are the

weakest type of bond.

You might think that DNA

should be strongly held
together- but it does
need to unzip- and
quite often!

DNA comes apart during

DNA Replication.

DNA REPLICATION

Recall that DNA is found in the

nucleus of all cells.

In order to make more cells (which

you are constantly doing), you
must make a copy of DNA first!

DNA Replication occurs during the

synthesis phase of the cell cycle
(before the cell actually divides).

DNA REPLICATION- STEP 1

The first step required in order

for DNA to make a copy of
itself is to break those
hydrogen bonds between the
bases.

An enzyme called DNA helicase

breaks the hydrogen bonds
and unzips the original parent
DNA molecule.

Multiple Choice

In a molecule of double-stranded DNA, the amount of Adenine present is always equal to the amount of

• cytosine

• guanine

• thymine

• uracil

Multiple Choice

The enzyme that unzips the DNA to prepare for replication

• helicase

• replicase

• polymerase

• synthase

DNA REPLICATION- STEP 2

Once the DNA strands are

unzipped, the nucleotides are
exposed.

The second step involves another

enzyme called DNA
Polymerase. This enzyme reads
the DNA and determines which
NEW nucleotides to add to the
parent strand.

1

2

1

3

Multiple Choice

Why does DNA replicate?

• To create a duplicate copy in case the original gets damaged

• New cells will have a copy of the DNA

• To create a new copy to replace the damaged original

Multiple Choice

The picture below shows an enzyme unwinding and unzipping DNA.

​

​What is the name of this enzyme?

• DNA Helicase

• DNA Ligase

• DNA polyermase

• RNA Polymerase

Multiple Choice

Which is the role of DNA polymerase?

• Unwinds the original DNA strand

• Lays down RNA primer for new nucleotide bases.

• Builds new strands by bringing in new nucleotide bases.

• Remove Okazaki fragments for new nucleotides.

REPLICATION FORKS

DNA is a very long molecule that must

be tightly coiled and packed into our
cells.

If the enzymes had to go from one end

of DNA all the way to the other, it
would take too long!

Replication forks form at multiple points in the DNA to speed

up replication.

2

Replication fork

3

Two replication forks make replication “bubbles”.

5’ AND 3’

Since DNA is a 3-Dimensional molecule made of linked

nucleotides, it really doesn’t have a “left” or “right”; “up” or
“down”.

If we have to refer to DNA’s direction we use 5’ and 3’ (5 prime

and 3 prime).

Recall that deoxyribose is a 5-carbon sugar. These numbers (5,3)

are in respect to the position on the 5-carbon sugar.

ANTIPARALLEL

DNA molecules are antiparallel- meaning the
two strands run parallel to one another, but in
different directions.

(It always looks like one strand is up-side down
relative to the other).

Multiple Choice

The two strands of DNA run in opposite directions. This is called

• Double helix

• Anti parallel

• Anti perpendicular

• double stranded

During DNA replication, DNA polymerase

READS the parent molecule in the 3’ 🡪 5’
direction.

New DNA is synthesized in the 5’ 🡪 3’

direction (opposite).

(How to Remember? When you READ a book

you would read chapters 3 to 5)

LEADING AND LAGGING DAUGHTER

STRANDS

DNA Polymerase moves from 3’ to 5’
One new strand will move continuously TOWARD the replication
fork- this is known as the leading strand.
Because the strands are anti-parallel, the other strand will move
AWAY from the replication fork- this is the lagging strand.

LEADING AND LAGGING DAUGHTER

STRANDS

The leading strand has continuous replication - it goes along
with the replication fork.

The lagging strand has discontinuous replication- it moves
against the replication fork.

LAGGING STRAND

Since the lagging strand is traveling away from the fork, as the
fork continues to open up, the lagging strand needs to jump
backwards to adjust (discontinuous).
Okazaki fragments are the short segments of new DNA on the
lagging strand.

Replication fork

FINAL PRODUCT- DNA

REPLICATION

The final product of DNA replication is two

molecules of DNA (4 strands total since each
molecule is double stranded).

However, it would not be appropriate to call the

molecules “new”.

DNA Replication is semi-conservative (semi=

half; conserve= to save)

Each time DNA is copied, the original DNA

molecule is saved. DNA is never destroyed
during replication!

Each new molecule
consists of one
parental strand, and
one (new) daughter
strand.

Multiple Choice

DNA Replication is

• conservative

• semiconservative

• liberal

• dispersive

Overview

● The process of DNA replication ensures that every

new cell that results from mitotic division has identical
DNA. Enzymes facilitate the replication process:

○ The first enzyme unzips the two strands of DNA that

compose the double helix, separating paired bases.

○ Each base that is exposed can only bond to its

complementary base.

○ Each of the separated strands serves as a template for

the attachment of complementary bases, forming a new
strand, identical to the one from which it was
“unzipped,” resulting in two identical DNA molecules.

SELF CHECK QUIZ

1.

The letters D.N.A. stand for ___________________________.

2.

DNA is shaped like a _______ _______.

3.

The four nitrogen bases are: adenine, ___________, _________,
____________.

4.

Adenine always bonds with ____________.

5.

Cytosine always bonds with ____________.

6.

7.

DNA replication is _______- _________________.

9. DNA replicates using specific [enzymes | carbohydrates].

10. Thymine and cytosine are [purines | pyrimidines].

11. Nitrogen bases are paired together using [hydrogen | covalent] bonds.

​DNA is important because it determines your physical _______________.

WHY DNA IS IMPORTANT:

DNA is important because it holds the “recipe” for

making proteins.

Your entire body is made out proteins!

DNA is your personalized instruction manual and yours

is unique to you (though everyone in this room
shares about 99% of the same DNA, that’s what
makes us human!)

DNA is very important; it controls

the workings of the cell. However,

it is trapped inside the nucleus.

(Like a mob boss in jail?)

In order to get all of its
instructions to the rest of
the cell, DNA relies on
its trusty sidekick....

(R.N.A.)

RIBONUCLEIC ACID

R.N.A. is also a nucleic acid- it is made out of linked

nucleotides (like DNA). Recall that nucleotides are
made of a sugar, phosphate, and nitrogen base.

DNA VS. RNA

RNA and DNA are very similar, but

there are some differences.

First of all, DNA is
double stranded, and
RNA is single
stranded. This means
that RNA is SMALLER
than DNA.

RNA contains 4 nitrogen bases: adenine, guanine,

cytosine and URACIL.

*Thymine is NOT present in RNA.

Uracil is
complementary to
adenine in DNA. It
essentially takes the
place of thymine.

The last major difference between

DNA and RNA is that RNA
contains the 5-carbon sugar
ribose. (Recall DNA contains
deoxyribose).

Ribose has one more
oxygen atom than
deoxyribose.

Ribose

Deoxyribose

RECAP

RNA is single stranded, so it is smaller than

DNA. This means it can leave the nucleus
(which DNA cannot).

RNA contains the sugar ribose.

RNA has 4 bases: A, T, C, and U. The base

pairing rules are as follows:

C pairs with G
G pairs with

C

A pairs with

U

U pairs with A

NO thymine in RNA

3 TYPES OF RNA

RNA’s job is to help DNA make proteins.

DNA must deliver its code to the remainder

of the cell- it relies on 3 molecules:

1)

Messenger RNA (mRNA)

2)

Transfer RNA (tRNA)

3)

Ribosomal RNA (rRNA)

MESSENGER RNA

mRNA is complementary to the original strand of

DNA. mRNA is first created in the nucleus and
then travels to the ribosomes out in the
cytoplasm. mRNA uses the DNA’s code (or
message) to make proteins!

Example)DNA Strand: G GCT T A

mRNA strand: C C GA A U

PROTEINS

Recall that proteins are made up of smaller parts called

amino acids. Another word for protein is “peptide”.

Individual DNA codes are
called “codons”. The codons
correspond to specific amino
acids. mRNA also has codons,
which are complementary to
DNA codons.

CODONS

Codons consist of groups of 3 nucleotides called triplets.

Example) DNA codon:
cytosine-cytosine-adenine

(CCA for short)

Each codon codes for one amino acid. This is

where we need RNA’s help.

C

C

A

DNA Template Strand:

A C G T

T

A

G C C

mRNA strand:

U G C A

A

U C G G

mRNA is always complementary to the template DNA

strand.

How many codons are there?

What does the other DNA strand look like?

Three DNA codons are transcribed into

three mRNA codons. mRNA codons
are specific to amino acids.

This is the beginning step of PROTEIN

SYNTHESIS.

Protein= well, protein

synthesis= to

make

DNA Template Strand: A C G T T A G C C

mRNA strand: U G C A A U C G G

1)

UGC

2)

AAU

3)

CGG

Which three
amino acids do
these mRNA
codons code for?

Though there are only 20 different amino acids, they are
sequenced differently and come in different shapes to
make for thousands of different proteins.

PROTEIN SYNTHESIS

• A two part process in which DNA is decoded into

corresponding proteins

• The first process is known as transcription

• The second process is translation

• Occurs in the nucleus and cytoplasm

TRANSCRIPTION

Transcription is the first part of

protein synthesis.

During transcription, mRNA is

created by transcribing the
DNA’s code.

Transcription occurs in the

nucleus.

(That’s where the DNA is!)

TRANSCRIPTION

During transcription,

the enzyme RNA
polymerase
temporarily unzips
DNA and adds
complementary RNA
nucleotides to the
growing mRNA
strand.

TRANSCRIPTION

Recall that mRNA is the messenger. It copies DNA’s code (or

“message”; “instructions”) and it is now responsible for
delivering this message to the rest of the cell.

Once the mRNA strand is completed, it leaves the nucleus (exits

via nuclear pores).

Transcription is complete.

(No protein yet...next stop, the ribosomes!)

TRANSLATION

Translation is the final step of protein synthesis- it involves

ALL THREE types of RNA (mRNA, tRNA and rRNA).

• Translation is a process in which the mRNA that was

manufactured during transcription is translated into an
amino acid sequence (proteins)

• occurs in the cytoplasm, on the ribosomes

Ribosomal RNA (rRNA)= a major

component of ribosomes; also helps
bond amino acids together to make
polypeptides (proteins)!

The goal of the 3 types of RNA is to
work together to make proteins using
the DNA’s instructions!

TRANSLATION

Transfer RNA (tRNA)= helps transfer amino acids to the corresponding

mRNA codons (tRNA is always complementary to the mRNA strand)

mRNA codons: U G C A A U C G G

tRNA anticodons: A C G U U A G C C

tRNA bases are referred to as “anti-codons” because they are complementary

to mRNA codons.

mRNA strand (codons) U G C A A U C G G

tRNA strand (anti-codons A C G U U A G C C

Example)

If the mRNA codon is

CUU, that would
translate to the amino
acid leucine.

The tRNA molecule that

will deliver leucine to
the ribosome has the
anticodon GAA.

Once the amino acid is delivered, the tRNA releases itself
from the ribosome, and leaves to find another amino acid to
add to the growing protein chain.

ANTICODON

LEUCINE

TRANSLATION

tRNA transfers amino acids to the

ribosome. The amino acids are attached
to the tRNA via a specialized enzyme
called tRNA synthetase.

Analogy: tRNA is a librarian; mRNA
codons are the book codes; amino acids
are the books. tRNA reads the mRNA and
fetches the appropriate amino acid.

LABEL THE FOLLOWING:
MRNA, TRNA, RIBOSOME,
PEPTIDE (PROTEIN) CHAIN,

AMINO ACID

TRANSLATION VIDEO

http://www.youtube.com/watch?v=B6O6uRb1D38

PUTTIN’ IT ALL TOGETHER

DNA codes for proteins
Proteins are made during

protein synthesis

Transcription occurs in the

nucleus. The final product of
transcription is an mRNA
strand.

Translation occurs on the

ribosomes in the cytoplasm.
The final product of
translation is a protein.

CODONS

There are 64 possible

codons:

43 = 64

64 codons for 20 amino acids.

Can more than one codon specify the same amino acid?

4 possible bases

(A, T, C or G)

3 bases in a codon

(triplets)

START AND STOP SIGNALS

Proteins are made up of a very specific

sequence of amino acids.

DNA contains “start” and “stop” codons so

that the cell knows where to start
decoding proteins and where to stop.

The start codon= AUG (codes for the

amino acid methionine)

Stop codons= UAA, UAG, UGA

VIDEO: PROTEIN SYNTHESIS

https://www.youtube.com/watch?v=h5mJbP23Buo