Mr. ColemanBiology

DNA- deoxyribonucleic acidDNA- deoxyribonucleic acid• DNADNA is often called

the blueprint of life..• In simple terms,

DNA contains the instructions for making proteins within the cell.

Why do we study DNADNA?

We study DNA for many reasons, e.g.,

• its central importance to all life on Earth,

• medical benefits such as cures for diseases,

• better food crops.

Chromosomes and DNAChromosomes and DNA

• Our genes are on our chromosomes.

• Chromosomes are made up of a chemical called DNA.

• Chromosomes are coiled DNA that is wrapped around a protein called a histone

Structure of Chromosomes• Chromatid: each half of a chromosomes

• Centromere: where the two chromatids of a chromosome join

• When a cell is not undergoing replication, the chromosome is not so tightly coiled. This is called chromatin.

Chromosomes in the cell• Human and animal cells are categorized as sex chromosomes or

autosomes:– Sex chromosomes: chromosomes that determine the sex of the

organism (XX, XY)• Sperm and egg cell only have ½ the number of chromosomes

– Haploid (n): 1 copy of each gene– Autosomes: all other chromosomes

• Homologous chromosomes: each parent donates one chromosome. This makes 2 copies of each chromosome (homologous)

– Diploid (2n): 2 copies of each gene

EXAMPLE

• Humans have 46 chromosomes– This is the total number and is called diploid (2n)– These 46 come from 23 pairs of chromosomes

(homologous chromosomes)• 22 pairs are autosomes

• 1 pair are the sex chromosomes

– If you only look at half of the chromosomes, this is the haploid number (1n)

The Shape of the MoleculeThe Shape of the Molecule

• DNA is a very long polymer.

• The basic shape is like a twisted ladder or zipper.

• This is called a double helix.

WHO CAME UP WITH THE STRUCTURE OF DNA?

• Watson and Crick– Used a big model (like

when you used the model kits)

– Were stuck until they were able to see the X-ray photos created by Franklin

– This gave them the information needed to determine a double helix

• Franklin– Did a procedure known

as X-ray diffraction

– Was able to determine a photo of the DNA molecule, but couldn’t quite figure out how the molecules bound together

The Double Helix MoleculeThe Double Helix Molecule

• The DNA double helix has two strands twisted together.

• (In the rest of this unit we will look at the structure of one strand.)

One Strand of DNAOne Strand of DNA• The backbone of

the molecule is alternating phosphate and deoxyribose, a sugar, parts.

• The teeth are nitrogenous bases.

phosphate

deoxyribose

bases

NucleotidesNucleotides

C C

C

OPhosphate

O

CC

O -P OO

O

One deoxyribose together with its phosphate and base make a

nucleotide.

Nitrogenous base

Deoxyribose

One Strand of DNAOne Strand of DNA

• One strand of DNA is a polymer of nucleotides.

• One strand of DNA has many millions of nucleotides.

nucleotide

Four nitrogenous basesFour nitrogenous bases

• Cytosine C• Thymine T

• Adenine A

• Guanine G

DNA has four different bases:

Two Kinds of Bases in DNA

• Pyrimidines are Pyrimidines are single ring bases.single ring bases.

• Purines are double Purines are double ring bases.ring bases.

C

C

C

C

N

N

OO

N

CC

CC

N

N

N

N

N

C

Thymine and Cytosine are Thymine and Cytosine are pyrimidinespyrimidines

• Thymine and cytosine each have one ring of Thymine and cytosine each have one ring of carbon and nitrogen atoms.carbon and nitrogen atoms.

C

C

C

C

N

N

OO

N

cytosinecytosine

C

C

C

C

N

N

OO

OO

thyminethymine

C

Adenine and Guanine are Adenine and Guanine are purinespurines

• AdenineAdenine and and guanineguanine each have two each have two rings of carbon and nitrogen atoms.rings of carbon and nitrogen atoms.

C

C

C

C

N

N

N

AdenineAdenine N

N

C

C

C

C

C

N

N

OO

N

GuanineGuanine N

N

C

Two Stranded DNATwo Stranded DNA• Remember, DNA

has two strands that fit together something like a zipper.

• The teeth are the nitrogenous bases but why do they stick together?

C

C

C

C

N

N

OO

N

C

C

C

C

N N

OO

N

N

N C

Hydrogen BondsHydrogen Bonds

• The bases attract each other because of hydrogen bonds.

• Hydrogen bonds are weak but there are millions and millions of them in a single molecule of DNA.

• (The bonds between cytosine and guanine are shown here.)

Hydrogen Bonds, Hydrogen Bonds, cont.

• When making hydrogen bonds, cytosine always pairs up with guanine,

• And adenine always pairs up with thymine.

• (Adenine and thymine are shown here.)

C

C

CC

N

N

N

N

N

C

C

C

C

C

N

N

OO

OO

C

Important:Important:

• Adenine and Thymine always join together in a double bond

A T

• Cytosine and Guanine always join together in a triple bond

C G

DNA by the numbersDNA by the numbers• Each cell has about 2 m

of DNA.• The average human has

75 trillion cells.• The average human has

enough DNA to go from the earth to the sun more than 400 times.

• DNA has a diameter of only 0.000000002 m.

The earth is 150 billion mor 93 million miles from the sun.

DNA Replication

• When cells reproduce, then need to be able to pass the “cell template” to the new cell they are creating

• In order to accomplish this, they need to make an entirely new copy of the genes within the nucleus.– DNA Replication: the process by which DNA in

a cell is copied before it undergoes cell division

DNA Replication

• Since DNA is in a double helix, it is first necessary to separate the two strands from each other– This is done by an enzyme known as a helicase– The helicases “unzip” the DNA double helix– The point at which the helicases work is called

the replicating fork

DNA Replication

• Once the DNA is separated, complementary base pairs bind to the now separated DNA strands– The enzymes called DNA polymerases are

used to facilitate this process– This process is creating two identical strands

• One strand is from the original double helix

• The other strand is being newly created by the DNA polymerases

DNA Replication

• When creating the new “daughter” strands of DNA, there is a gap at the replicating fork– This area does not have enough room for the

DNA polymerases to work– These “gaps” are filled in later by an enzyme

called DNA ligase

DNA Replication summary

• DNA replication creates two completely identical double helix strands of DNA

• Each double helix contains 1 original strand of the original DNA strand and 1 newly created strand of DNA– This is called semi-conservative replication

ERRORS IN REPLICATION

• For every billion base pairs that are created, there is an error in replication (one base is added to the strand or deleted that shouldn’t be)

• The reason for this low error is because the DNA polymerase has a repair function– The DNA is “proofread” as it is created– If an error is detected, the DNA polymerase

can repair the error

ERRORS IN REPLICATION

• Despite the high degree of accuracy in DNA replication, errors do occur

• An error in replication is called a mutation– Some errors do not have a significant impact on the

survival of the organism (the mutation is not expressed)– Others can have serious effects depending on what

proteins the affected gene create• Example: Cancer is a disease when a cell can’t stop

dividing . The stopping mechanism was damaged. Uncontrolled cell division creates tumors and can seriously affect the physiology of the organism.

TYPES OF ERRORS

• Point mutation: When one base is altered– Insertion: When a base is accidentally added to

the DNA strand.– Deletion: When a base is accidentally removed

from the DNA strand– Substitution: When one base is accidentally

replaced by another

• All of these can cause a frame-shift mutation which alters protein synthesis

Eukaryotic vs. Prokaryotic

• In prokaryotic cells (bacteria), there is only 1 circular chromosome

– DNA replication is a little different from eukaryotic cells

• Two replicating forks are created and proceed in opposite directions

• Replication continues until the forks meet and you now have 2 new, identical, chromosomes

PROTEIN SYNTHESIS

• In the cell, proteins have many uses:– Works as an enzyme– Is one of the components of many organelles

and structural aspects of the cell

• Therefore, the ability to create proteins is essential for proper cell function

• The process to create proteins is called protein synthesis

PROTEIN SYNTHESIS

• The blueprint for all proteins exists in DNA

• The flow of information from DNA to the creation of proteins goes as follows:

DNA mRNA tRNA/rRNA (ribosomes)

transcription translation

COMPONENTS OF PROTEIN SYNTHESIS

• In addition to the DNA template, there are 3 other molecules necessary in protein synthesis:– mRNA (messenger RNA)– tRNA (transfer RNA)– rRNA (ribosomal RNA)

• Each type of molecule is necessary for a step in protein synthesis

TRANSCRIPTION

DNA is copied into a complementary strand of mRNA.

WHY?

• DNA cannot leave the nucleus. Proteins are made in the cytoplasm. mRNA serves as a “messenger” and carries the protein building instructions to the ribosomes in the cytoplasm.

TRANSCRIPTION

• To transcribe means to write something down

• In the first phase of protein synthesis, the DNA code is “written down” onto a form of ribonucleic acid known as messenger RNA (mRNA)– mRNA is a single strand assembly of

nucleotides

TRANSCRIPTION

• We already discussed the base pairing that occurs between nucleotides:

• A=T (double bond)• C=G (triple bond)

• The way that DNA is “written down” is similar to the way that DNA is replicated– A segment of DNA is opened using RNA polymerase

enzymes– Complementary nucleotides bind to the DNA template to

create the mRNA

TRANSCRIPTION

• There is one substitution:

– Thymine in DNA is replaced by Uracil (U) in mRNA

– Therefore, the binding in mRNA goes as follows:

• A=U (double bond)

• G= C (triple bond)

PROCESS OF TRANSCRIPTION

• Transcription starts at a specific DNA site. This is called the promoter.– This is where the RNA polymerase binds

• The RNA polymerase then adds complementary bases to the DNA strand– Again, T is replaced by U

• Finally, the RNA polymerase hits a DNA site that causes transcription to end. This is called the termination signal

TRY THIS

• If you have the following DNA sequence, what would be the mRNA strand created:

CGATTTGCTACA

CODONS

• What does the mRNA code tell you?– A sequence of 3 bases makes up a codon– The codon codes for a specific amino acid

• The handout tells you what amino acids each codon codes for

TRANSLATION

PROTEINS

Structural and functional components of organisms.

• Composed of amino acids

• order of nucleotides in DNA determines order of amino acids in a protein

• One gene codes for one protein

TRANSLATION

• To translate something means to convert it from one language/code to another

• In translation, you take the mRNA strand and use it to create the amino acid sequence to make a protein.

• Translation uses:– tRNA (transfer): carries amino acids to the ribosome.– rRNA (ribosomal): composes the ribosome.

TRANSLATION

GENETIC CODEThe “language” that translates the sequence of nitrogen

bases in DNA (mRNA) into the amino acids of a protein.

• Codon = three nucleotides on mRNA• Anticodon = three nucleotides on tRNA• One codon specifies one amino acid• Some codons are redundant (code for the same amino

acid)• The genetic code is universal to all organisms

AMINO ACID

DNA: TAC CTT GTG CAT GGG ATC

mRNA AUG GAA CAC GUA CCC UAG

A.A MET G.A HIS VAL PRO STOP

AMINO ACIDS

IMPORTANT CODONS

• AUG = start translation (Met)

• UAA, UAG, UGA= stop translation

TRANSLATION

1. mRNA binds to the ribosome.

2. Ribosome searches for start codon (AUG)

3. tRNA brings correct amino acid (methionine) to the ribosome.– Each tRNA carries one type of amino acid. – The anticodon (three nitrogen bases on tRNA)

must complement codon for amino acid to be added to protein chain

TRANSLATION

4. ribosome reads next codon

5. tRNA’s continue lining up amino acids according to codons

6. peptide bonds link amino acids together

7. ribosome reaches STOP codon

• Amino acid chain is released

PROTEIN SYNTHESIS

• This is a molecule of messenger RNA.

• It was made in the nucleus by transcription from a DNA molecule.

A U G G G C U U A A A G C A G U G C A C G U U

mRNA molecule

codon

PROTEIN SYNTHESIS

• A ribosome on the rough endoplasmic reticulum attaches to the mRNA molecule.

A U G G G C U U A A A G C A G U G C A C G U U

ribosome

A U G G G C U U A A A G C A G U G C A C G U U

It brings an amino acid to the first three bases (codon) on the mRNA.

Amino acid

tRNA molecule

anticodon

U A C

A transfer RNA molecule arrives.

The three unpaired bases (anticodon) on the tRNA link up with the codon.

A U G G G C U U A A A G C A G U G C A C G U U

Another tRNA molecule comes into place, bringing a second amino acid.

U A C C C G

Its anticodon links up with the second codon on the mRNA.

A U G G G C U U A A A G C A G U G C A C G U U

A peptide bond forms between the two amino acids.

Peptide bond

C C G U A C

A U G G G C U U A A A G C A G U G C A C G U U

The first tRNA molecule releases its amino acid and moves off into the cytoplasm.

C C G U A C

A U G G G C U U A A A G C A G U G C A C G U U

C C G

The ribosome moves along the mRNA to the next codon.

A U G G G C U U A A A G C A G U G C A C G U U

Another tRNA molecule brings the next amino acid into place.

C C G

A A U

A U G G G C U U A A A G C A G U G C A C G U U

Another tRNA molecule brings the next amino acid into place.

C C G A A U

A U G G G C U U A A A G C A G U G C A C G U U

The polypeptide chain gets longer.

G U C

A C G

The process continues.

This continues until a termination (stop) codon is reached.

The polypeptide is then complete.