CHAPTER 10

DNA STRUCTURE AND REPLICATION

1. DNA2. DNA Replication3. From DNA to RNA to Protein4. Mutations

1. DNA

DNA• Has the capacity to store genetic information.

• Can be copied and passed from generation to

generation.

DNA is a nucleic acidIt consists on a long chain of molecules called nucleotides.

The nucleotides are joined by a sugar-phosphate backbone.

4 nucleotides

Differ in their nitrogenous bases

Thymine (T) Cytosine (C) Adenine (A) Guanine (G)

!

2 DNA strands join together to form a double helix.

AT

GC

T A

C G

Hydrogen Bond

ONLY PAIRS FORMED BETWEEN A-T AND G-C

James WATSON and Francis CRICK determined that DNA is a double helix.

2. DNA Replication

2. DNA Replication

When a cell or whole organism reproduces, a complete set of genetic instructions must pass from one generation to the next.

DNA is split in 2 strands and each strand is copied to form a new DNA molecule

DNA replication:

- starts at specific sites (origin of replication)

- goes in both directions

3. From DNA to RNA to PROTEIN

From DNA to RNA to PROTEIN

How an Organism’s GenotypeProduces Its Phenotype

GENOTYPE: is the Genetic Constitution of an organism or cell, its genetic make up. It is the sequence of nucleotide bases in DNA

PHENOTYPE: is the organism’s specific traits, the organism’s total physical appearance and constitution. It is the result of the actions of proteins.

GENOTYPE: eye color gene called: bey2

PHENOTYPE:

Example for phenotype and genotype

Nucleus

Cytoplasm

DNA specifies the synthesis of proteins in 2 stages:

DNA

Protein

2. TRANSLATION

1. TRANSCRIPTION

RNA

TRANSCRIPTION: DNA RNA

TRANSLATION: RNA PROTEINFrom Nucleic Acid to Amino Acid

Different Molecules - Different Languages

1 gene – 1 polypeptide hypothesis:

The function of an individual gene dictates the production of a specific polypeptide.

CODONS:triplets of bases

Each codon corresponds to an

Amino Acid

DNA has 2 strands.4 bases combining in pairs in DNA:

A-TG-C

RNA has 1 strand.4 bases combining in pairs in DNA:

A-UG-C

The genetic code is shared by all organisms.

GENETIC CODE:It is the set of rules relating nucleotide

sequence to amino acid sequence.

Transcription:From DNA to RNA

In transcription:•Genetic information is transferred from DNA to RNA.

•An RNA molecule is transcribed from a DNA template.

Transcription done by RNA Polymerase which adds RNA nucleotide 1 by 1 to the newly made RNA sequence. The base of the nucleotide is complementary to the DNA base.

RNA polymerase DNADNA of a gene

Promoter Terminator

RNA

Initiation

Elongation

Completed RNASingle strand

Termination

1. INITIATION:

The “start transcribing” signal is a nucleotide sequence called a promoter.

The first phase of transcription is initiation:

RNA polymerase attaches to the promoter.

RNA synthesis begins.

2. ELONGATION:Second phase. The RNA gets longer.

3. TERMINATION:

RNA polymerase reaches a sequence of DNA bases called a terminator.

3 PHASES OF TRANSCRIPTION

But RNA is not ready yet…

It needs to be processed.

The eukaryotic cell processes the RNA after transcription by:

The Processing of Eukaryotic RNA

•Adding a cap and tail

•Removing introns

•Splicing exons together

What does this mean?

What are caps and tails?

A cap and a tail are added at the extremities of the newly RNA molecule.

It protects the RNA from enzymes;and help the ribosomes recognize it.

A gene contains EXONS and INTRONS

Exon Exon ExonIntron Intron

What are EXONS and INTRONS?

EXONS are kept and will be translated into polypeptides(EX for EXiting the nucleus)

INTRONS are rid of.

Remember…

1 gene – 1 polypeptide hypothesis:

The function of an individual gene dictates the production of a specific polypeptide.

ACTUALLY, it could be several…..1 gene can give different versions of a protein.

By combining (splicing) different exons.

Exon Exon ExonIntron Intron

DNA

RNA

Transcription

Cap and Tail added

Introns removed

Splicing of exons

Coding sequence READY TO GO!

Translation:From RNA to PROTEIN

From nucleic acid to amino acid

THE PLAYERS:

Messenger RNA (mRNA):- carries the coding sequence to be translated into a protein

Transfer RNA (tRNA): The translator.- Each carries a specific amino acid (aa)- Matches it with codons in mRNA using anticodons

Ribosomes:- 2 subunits- made of proteins and ribosomal RNA (rRNA)- brings tRNA and mRNA together. Has 2 sites and can hold 2 tRNA.- catalyze the peptide bonds to form polypeptide

aa

aa

codons

ribosome

mRNA

tRNA

Growing polypeptide

Next amino acid to be added

Formation of a Protein

Translation: The Process

Translation is divided into 3 phases:

1. Initiation

2. Elongation

3. Termination

1. Initiation– The mRNA (recognized by its cap and tail)– The first amino acid with its attached tRNA (the first codon

is always the same (AUG), it marks the starts of the coding sequence and is therefore called start codon)

– The two subunits of the ribosome

– Step 1, codon recognition• The anticodon of an incoming tRNA pairs with

the mRNA codon.

– Step 2, peptide bond formation• The ribosome catalyzes bond formation

between amino acids. The polypeptide grows.

– Step 3, translocation• A tRNA leaves the P site of the ribosome.• The ribosome moves down the mRNA.

2. Elongation3 steps:

codons

Empty site

New codonmRNA moves

1. codon recognition 2. peptide bond formation

3. translocation

3. Termination

Elongation continues until the ribosome reaches a stop codon and the polypeptide is released.

Review:DNA RNA Protein

4. Mutations

A mutation is any change in the nucleotide sequence of DNA.

Mutations

Types of Mutations1. A base can be replaced by another one2. A base can be inserted3. A base can be removed

Any of these change the reading frame of the genetic message

which can result in changes in the amino acids in proteins

and therefore change the function of the protein

can be disastrous

• Errors in DNA replication.

• Physical or chemical agents called mutagens.

Mutations may result from

Although mutations are often harmful,

They are also the source of the rich diversity of genes in the living world.