DNA

Replication, Transcription and Translation

DNA- stands for deoxyribonucleic acid

DNA is a combination of nucleic acids and histones

•nucleic acid- polymer of nucleotides

•histone- protein structure

Nucleotides are made of 3 parts:

1. Phosphate group2. A 5-carbon (pentose) sugar,

deoxyribose3. A nitrogenous base

The nitrogenous bases:1. Adenine2. Guanine3. Thymine4. Cytosine

Purines

Pyrimidines

The combination of the phosphate, sugar, and nitrogenous base makes a nucleotide

Because of the four nitrogenous bases, four different nucleotides are seen in DNA

The covalent and hydrogen bonding between nucleotides creates the DNA molecule

DNA MoleculeDNA is made of two strands of nucleotides. Together, the two strands form a “ladder” or double helix.

The sugar-phosphate backbone forms the sides of the ladder

Phosphates

Sugars

“Sides of the ladder”

The two strands are identical to each other, but antiparallel (heading in opposite directions)

The nitrogenous bases of the two strands forms the rungs.

Hydrogen bonding between the bases holds the chains together.

A - T C - GThe random order of the bases down the DNA chain accounts for unlimited number of proteins made from DNA

2 hydrogen bonds

AT

2 hydrogen bonds

3 hydrogen bonds

G

C

3 hydrogen bonds

Individual nucleotides are bonded together through condensation reactions between Carbon 3 and the next phosphate group

OH H

P

OH

OH

O CH2O

3’

5’

LEAVES TO FORM H2O

P OO

OH

OCH2

3’

5’ OH H+

During interphase of the cell cycle, DNA makes an exact copy of itself. This process is DNA Replication

The process involves the separation or “unzipping” of the DNA chains.

Each strand of the original chain serves as a template to assemble the new complementary strand.

Result is two identical DNA chains each with an original template from old DNA

Replication Bubbles

The process of having DNA replication with one strand being old DNA (template) and one new replicated DNA (complementary strand) is called semiconservative replication

Original DNA strand

Templates after “unzipping”

New DNA strands attach to templates

Major steps in DNA Replication:

Helicase enzyme breaks hydrogen bonds between DNA strands

The location where the helicase is splitting the DNA chain is the Replication Fork

Helicase Replication Fork

DNA is divided into two individual strands.

One strand is 3’ 5’, the other is 5’ 3’.

5’

5’

3’

3’

DNA polymerase, an enzyme, only attaches and moves down the 3’5’ strand.

It attaches new nucleotides to the exposed template in the 5’3’ direction, creating a new antiparallel strand

On the 3’ 5’ strand, the DNA Polymerase follows the replication fork.

This allows the 5’ 3’ strand to be made in a long continuous chain. This is the leading strand

Leading strand DNA polymerase

Lagging strand

On the original 5’ 3’ template, DNA polymerase must operate in the opposite direction away from the replication fork.

This strand is the lagging strand

Leading strand DNA polymerase

Lagging strand

This results in DNA polymerase attaching many short segments of nucleotides away from the replication fork.

These short segments are called Okazaki fragments

The Okazaki fragments are connected together by an enzyme called DNA Ligase.

All of the short segments are joined into one long complementary strand.

After replication, DNA polymerase and other enzymes proofread the new strand for errors.

If an error is not corrected, it becomes a mutation.

Mutation types:

1. Substitution mutation-

Incorrect nucleotide inserted

AT

AG

GC

GC

AT

AT

GC

GC

2. Insertion mutation-

an extra nucleotide inserted into the sequence

G

3. Deletion Mutation

a nucleotide is removed or missing

AT

AGC

GC

RNA stands for- ribonucleic acid

Major differences between DNA and RNA:

1. Sugar in the nucleotide is ribose

HOH

DeoxyriboseHOCH2

OHOH

RiboseHOCH2 HOHO

2. RNA does not hold the nitrogenous base Thymine. Instead Uracil is used.

When base pairing occurs, Uracil bond to Adenine.

A-U

3. RNA is single stranded. It does not form a helix like DNA

There are 3 types of RNA

1. Messenger RNA (mRNA)

Single stranded RNA that is a copy of a small segment of DNA. It carries the genetic code from the DNA to the cytoplasm.

mRNA leaves the nucleus and becomes the template for protein synthesis.

mRNA attaches to a ribosome where amino acid attachment takes place.

2. Transfer RNA (tRNA)

Carries a specific amino acid to the ribosome for polypeptide (protein) synthesis.

3. Ribosomal RNA (rRNA)

Type of RNA that makes about 60% of a ribosome. Its exact function is not currently known.

TranscriptionmRNA

Transcription- process of making RNA from a small segment of DNA.

This mRNA carries the protein-building instructions to the ribosomes for protein production

Steps of transcription are similar to DNA Replication.

DNA (copy) is “unzipped” by RNA polymerase. RNA polymerase moves down the 3’ 5’ strand (sense strand) attaching RNA nucleotides.

3’ 5’

5’ 3’A

CG T

G A G AG

C T C TC

RNA polymeraseSense strand

“Unzipping”

3’ 5’

5’ 3’A

CG T

T A G AG

A T C T CC UUA

Notice RNA uses Uracil, not Thymine

mRNA

As the RNA is elongated in the 5’ 3’ direction, it detaches from the DNA template.3’ 5’

5’ 3’A

TT

C G C TA

G C G A TG UU

A

AGCG A

mRNA

Transcription ends when a termination sequence on the DNA sequence is reached.The mRNA is released and the two DNA strands reattach.

T AGC

TA

AT

GC G

CA T

G U A G CA U

mRNA

DNA

mRNA

DNA

Translation

mRNA

Protein

Translation- process of making a protein from the mRNA template.

Translation begins when mRNA attaches to a ribosome in the cytoplasm

The ribosome reads three nucleotides at a time on the mRNA. The three nucleotides are called a codon. The ribosome looks for the “start” codon- AUG. It does not attach until it finds AUG

Ribosomes are made in two parts: small subunit and large subunit.

Small subunit

Large subunitP siteA site

The small subunit is the first part to attach to the 5’ end of the mRNA chain when AUG is identified.

tRNA carrying an anticodon, the opposite of the codon, UAC, attaches to the codon

tRNA

AUG CGU GGUUAAUAC

CCG

The large subunit attaches to the small subunit.

The tRNA, still hydrogen bonded to the mRNA, is found in the P site of the ribosome.

AUG CGU GGUUAAUAC

The next tRNA matches with the next codon and bonds in the A site.

This causes the amino acids carried on the tRNA molecules to form a peptide bond.

AUG CGU GGUUAAUAC

Met.

GCA

Lys.

The ribosome then shifts down to the next codon which opens the A site for the next tRNA

This process continues until a stop codon is reached.

The stop codon causes the termination of translation.

The amino acid chain is released from the ribosome and the ribosome is released from the mRNA.