CH 25DNA

Monomer of Nucleic Acids

A molecular complex of three types of subunit molecules

1. Phosphate

2. Pentose sugar

3. Nitrogen-containing base

NUCLEOTIDES

DNA (deoxyribonucleic acid)

Stores genetic material

Codes for the order in which AA are joined to form a protein

RNA (ribonucleic acid)

Conveys DNA’s instructions regarding the amino acid sequence in a protein

3 types:

1. Messenger RNA

2. Ribosomal RNA

3. Transfer RNA

NUCLEIC ACIDS

DNA RNA

Sugar Deoxyribose Ribose

Bases Adenine, Guanine, Thymine, Cytosine

Adenine, Guanine, Uracil, Cytosine

Strands Double Stranded (with base pairing) Single Stranded

Helix Yes No

NUCLEIC ACIDS

DNA is double-stranded with

complementary base pairing

A-TC-G

DNA STRUCTURE

Purines:

Adenine and Guanine

Two rings

Found in DNA and RNA

Pyrimidines:

Cytosine, Thymine, Uracil

One ring

Cytosine found in DNA and RNA

Thymine found in DNA only

Uracil found in RNA only

NUCLEOTIDES

In the mid-1900s, scientists knew that chromosomes, made up of DNA (deoxyribonucleic acid) and proteins, contained genetic information.However, they did not know whether the DNA or

the proteins was the actual genetic material.

HISTORY

Various researchers showed that DNA was the genetic material when they performed an experiment with a T2

virus.

By using different radioactively labeled components, they demonstrated that only the virus DNA entered a bacterium

to take over the cell and produce new viruses.

HISTORY

VIRAL DNA IS LABELED

VIRAL CAPSID IS LABELED

The structure of DNA was determined by James Watson and Francis Crick in the early 1950s.

They deduced the following:• DNA has a twisted, ladder-like structure

(double helix)

• The sugar-phosphate molecules make up the sides of the ladder and the bases make up the

rungs

• Since A bonds with T and G with C, the rungs have a constant width

(purine paired with a pyrimidine)

DNA STRUCTURE

BASE PAIRING

TWO DNA STRANDS ARE ANTI-PARALLEL – THEY RUN IN OPPOSITE

DIRECTIONS.

DNA replication occurs during chromosome duplication;

an exact copy of the DNA is produced with the aid of

DNA polymerase (an enzyme)

Hydrogen bonds between bases break and enzymes “unzip” the molecule.

Each old strand of nucleotides serves as a template for each new strand.

REPLICATION

New nucleotides move into complementary positions are joined by DNA polymerase.

The process is semiconservative because each new double helix is composed of an old strand of

nucleotides from the parent molecule and one newly-formed strand.

REPLICATION

LADDER CONFIGURATION & DNA REPLICATION

1. ‘DNA: Structure, function and replication’ - WS

2. ‘DNA’ notes booklet3. ‘Protein Synthesis’ – WS

4. CH 2 Review Q’s

TO WORK ON:

25.2GENE EXPRESSION

A gene is a segment of DNA that specifies the amino acid sequence of a protein.

Gene expression occurs when gene activity leads to a protein product in the cell.

A gene does not directly control protein synthesis; instead, it passes its genetic information on to RNA, which is more

directly involved in protein synthesis.

GENE EXPRESSION

Bases: Adenine-Uracil, Cytosine-Guanine

Types:

1. Messenger RNA (mRNA): takes message from DNA to ribosome

2. Ribosomal RNA (rRNA): along with proteins, makes up the ribosomes – where proteins are

synthesized

3. Transfer RNA (tRNA): transfers amino acids to the ribosomes

RNA

RNA STRUCTURE

1. Transcription: makes an RNA molecule complementary to a portion of DNA

2. Translation: occurs when the sequence of bases of mRNA directs the sequence of amino

acids in a polypeptide

PROTEIN SYNTHESIS

DNA specifies the synthesis of proteins because it contains a triplet code: every three bases stand for

one amino acid.

Each three-letter unit of an mRNA molecule is called a codon.

Most amino acids have more than one codon; there are 20 amino acids with a possible 64 different

triplets.

The code is nearly universal among living organisms.

THE GENETIC CODE

MRNA CODONS

GENE EXPRESSION

During transcription in the nucleus, a segment of DNA unwinds and unzips, and the DNA serves

as a template for mRNA formation.

RNA polymerase joins the RNA nucleotides so that the codons in mRNA are complementary to

the triplet code in DNA.

TRANSCRIPTION

DNA contains exons and introns.

Before mRNA leaves the nucleus, it is processed and the introns are excised so that only the exons

are expressed.

The splicing of mRNA is done by ribozymes, organic catalysts composed of RNA, not protein.

Primary mRNA is processed into mature mRNA.

PROCESSING OF MRNA

Translation is the second step by which gene expression leads to protein synthesis.

During translation, the sequence of codons in mRNA specifies the order of amino acids in a

protein.

Translation requires several enzymes and two other types of RNA: transfer RNA and

ribosomal RNA.

TRANSLATION

During translation, transfer RNA (tRNA) molecules attach to their own particular amino

acid and travel to a ribosome.

Through complementary base pairing between anticodons of tRNA and codons of mRNA, the

sequence of tRNAs and their amino acids form the sequence of the polypeptide.

TRNA

TNA: AMINO ACID CARRIER

Ribosomal RNA, also called structural RNA, is made in the nucleolus.

Proteins made in the cytoplasm move into the nucleus and join with ribosomal RNA to form the

subunits of ribosomes.

A large subunit and small subunit of a ribosome leave the nucleus and join in the

cytoplasm to form a ribosome just prior to protein synthesis.

RRNA

A ribosome has a binding site for mRNA as well as binding sites for two tRNA molecules at a time.

As the ribosome moves down the mRNA molecule, new tRNAs arrive, and a polypeptide forms and

grows longer.

Translation terminates once the polypeptide is fully formed; the ribosome separates into two subunits

and falls off the mRNA.

Several ribosomes may attach and translate the same mRNA, therefore the name polyribosome.

RIBOSOMES

POLYRIBOSOME

During translation, the codons of an mRNA base-pair with tRNA anticodons.

Protein translation requires these steps:1) Chain initiation

2) Chain elongation3) Chain termination.

Enzymes are required for each step, and the first two steps require energy.

TRANSLATION: 3 STEPS

During chain initiation, a small ribosomal subunit, the mRNA, an initiator tRNA, and a large

ribosomal unit bind together.First, a small ribosomal subunit attaches to the

mRNA near the start codon.The anticodon of tRNA, called the initiator RNA,

pairs with this codon.Then the large ribosomal subunit joins.

CHAIN INITIATION

The initiator tRNA passes its amino acid to a tRNA-amino acid complex that has come to the

second binding site.

The ribosome moves forward and the tRNA at the second binding site is now at the first site, a

sequence called translocation.

The previous tRNA leaves the ribosome and picks up another amino acid before returning.

CHAIN ELONGATION

Chain termination occurs when a stop-codon sequence is reached.

The polypeptide is enzymatically cleaved from the

last tRNA by a release factor, and the ribosome falls away from the mRNA molecule.

A newly synthesized polypeptide may function alone or become part of a protein.

CHAIN TERMINATION

DNA in the nucleus contains a triplet code; each group of three bases stands for one amino acid.

During transcription, an mRNA copy of the DNA template is made.

The mRNA is processed before leaving the nucleus.

The mRNA joins with a ribosome, where tRNA carries the amino acids into position during

translation.

REVIEW

A gene mutation is a change in the sequence of bases within a gene.

MUTATIONS

Frameshift mutations involve the addition or removal of a base during the formation of mRNA; these change the genetic message by shifting the

“reading frame.”

FRAMESHIFT MUTATION

The change of just one nucleotide causing a codon change can cause the wrong amino acid to be inserted in a

polypeptide; this is a point mutation.

In a silent mutation, the change in the codon results in the same amino acid.

If a codon is changed to a stop codon, the resulting protein may be too short to function; this is a nonsense

mutation.

If a point mutation involves the substitution of a different amino acid, the result may be a protein that cannot reach

its final shape; this is a missense mutation.

An example is Hbs which causes sickle-cell disease.

POINT MUTATIONS

SICKLE CELL DISEASE

Mutations can be spontaneous or caused by environmental influences called mutagens.

Mutagens include radiation (X-rays, UV radiation), and organic chemicals (in cigarette smoke and

pesticides).

DNA polymerase proofreads the new strand against the old strand and detects mismatched

pairs, reducing mistakes to one in a billion nucleotide pairs replicated.

CAUSE & REPAIR OF MUTATIONS

Transposons are specific DNA sequences that move from place to place within and between

chromosomes.

These so-called jumping genes can cause a mutation to occur by altering gene expression.

It is likely all organisms, including humans, have transposons.

TRANSPOSONS: JUMPING GENES