Chapter 25, 26: Nucleotides, Nucleic Acids, Heredity, Gene expression, and Protein Synthesis

Chem 104K. Dunlap

The Molecules of Heredity– Each cell of our bodies contains thousands of different

proteins.– How do cells know which proteins to synthesize out of the

extremely large number of possible amino acid sequences?– From the end of the 19th century, biologists suspected that

the transmission of hereditary information took place in the nucleus, more specifically in structures called chromosomeschromosomes.

– The hereditary information was thought to reside in genesgenes within the chromosomes.

– Chemical analysis of nuclei showed chromosomes are made up largely of proteins called histoneshistones and nucleic nucleic acidsacids.

The Molecules of Heredity

– By the 1940s, it became clear that deoxyribonucleic acids (DNA)deoxyribonucleic acids (DNA) carry the hereditary information.

– Other work in the 1940s demonstrated that each gene controls the manufacture of one protein.

– Thus the expression of a gene in terms of an enzyme protein led to the study of protein synthesis and its control.

Nucleic Acids

• There are two kinds of nucleic acids in cells:1) ribonucleic acids (RNA)

2) deoxyribonucleic acids (DNA)

• Both RNA and DNA are polymers built from monomers called nucleotides.

• A nucleotide is composed of: – a base, a monosaccharide, and a phosphate.

Nucleic Acids

• made up of nucleotides• found in all living cells except RBC• deoxyribonucleic acid (DNA) and ribonucleic acid (RNA)• DNA is in the nucleus• RNA is in the cytoplasm

• function in the storage and transmission of genetic material•And control and direct all protein synthesis

1) Ribose and Deoxyribose

-each nucleotide contains1) a sugar2) a base3) phosphoric acid unit

RNA DNA

Bases in Nucleic Acids

Bases for DNA:A, G, C, T

Bases for RNA:A, G, C, U

Pyrimidine/Purine Bases

Formation of a Nucleotide

• Alternating phosphate, sugar molecules form the backbone• The rxn between phosphate and sugar forms an ester bond with the elimination of water• The sugar bonds with a base, forming tertiary amine, with the elimination of water

DNA and RNA Strand

• The sequence of the bases in DNA or RNA form the primary structure

DNA

Structure of DNA and RNA

DNA - 2° Structure

• Secondary structureSecondary structure:: the ordered arrangement of nucleic acid strands.– the double helix model of DNA 2° structure was

proposed by James Watson and Francis Crick in 1953.

• Double helixDouble helix:: a type of 2° structure of DNA in which two polynucleotide strands are coiled around each other in a screw-like fashion.

Watson and Crick

The DNA Double Helix

• Figure 25.4 Three-dimensional structure of a DNA double helix.

DNA Double Helix• Like a spiral staircase: -the phosphate sugar backbone represents the hand rail, the bases represent the steps

• Hydrogen bonding occurs between the bases….. For DNA: A bonds with T C bonds with G For RNA: A bonds with U C bonds with G

Base Pairing

Higher Structure of DNA– DNA is coiled around proteins called histones.histones.– Histones are rich in the basic amino acids Lys and

Arg, whose side chains have a positive charge.– The negatively-charged DNA molecules and

positively-charged histones attract each other and form units called nucleosomes.

– Nucleosome:Nucleosome: a core of eight histone molecules around which the DNA helix is wrapped.

– Nucleosomes are further condensed into chromatin.chromatin.

– Chromatin fibers are organized into loops, and the loops into the bands that provide the superstructure of chromosomes.chromosomes.

Chromosomes

Chromosomes

Chromosome

Chromosomes

• To accommodate the large amount of genetic material, DNA in the nuclei of cells is coiled around proteins called Histones

Micrograph of a chromosome

DNA and RNA

• The three differences in structure between DNA and RNA are:

– DNA bases are A, G, C, and TT; the RNA bases are A, G, C, and U.U.

– The sugar in DNA is deoxyribosedeoxyribose; in RNA it is ribose.ribose.

– DNA is always double strandeddouble stranded; there are several kinds of RNA, most of which are single-stranded.single-stranded.

RNA• RNA molecules are classified according to

their structure and function.

Genes, Exons, and Introns

Gene:Gene: A segment of DNA that carries a base sequence that directs the synthesis of a particular protein, tRNA, or mRNA.– There are many genes in one DNA molecule.

Exon:Exon: A section of DNA that, when transcribed, codes for a protein or RNA.

Intron:Intron: A section of DNA that does not code for anything functional.

Structure of tRNA

Replication of DNA

The DNA in the chromosomes carries out two functions:– (1) It reproduces itself. This process is called

replicationreplication.– (2) It supplies the information necessary to

make all the RNA and proteins in the body, including enzymes.

Replication begins at a point in the DNA called the origin of replication or a replication forkreplication fork.

The central dogma of molecular biology:

– Information contained in DNA molecules is expressed in the structure of proteins.

– Gene expression is the turning on or activation of a gene.

proteinTranscription Translation

DNAreplication

DNA mRNA

Reverse transcriptase

RNAreplication

DNA Replication

• The two strands of DNA in the helix are complementary

• When ready to replicate the two strands unwind

• Bases in the cell will migrate and bind with their complementary base to form an exact replica of the original

DNA Replication• ReplicationReplication involves separation of the two

original strands and synthesis of two new daughter strands using the original strands as templates.

– DNA double helix unwinds at a specific point called an origin of replication.origin of replication.

– Polynucleotide chains are synthesized in both directions from the origin of replication; that is, DNA replication is bidirectional.bidirectional.

DNA Replication• Unwinding the DNA double helix.

– Replication of DNA starts with unwinding of the double helix.

– Unwinding can occur at either end or in the middle.– Unwinding proteins called helicaseshelicases attach

themselves to one DNA strand and cause separation of the double helix.

DNA Replication

Transcription• Transcription:Transcription: the process by which

information encoded in a DNA molecule is copied into an mRNA molecule.– Takes place in the nucleus– Transcription starts when the DNA double helix

begins to unwind near the gene to be transcribed.– Only one strand of the DNA is transcribed.– Ribonucleotides assemble along the unwound

DNA strand in a complementary sequence.– Enzymes called polymerasespolymerases (poly)(poly) catalyze

transcription

TranscriptionThe information in one DNA strand is transcribed to a strand of RNA. The termination site is the locus of termination of transcription.

Transcription by RNA• First step in protein synthesis• The segment of DNA that contains the necessary information, unwinds, to expose the bases • The exposed bases, provide the template for messenger RNA (mRNA) synthesis

Determining the Sequence of Bases in RNA

Transcription

RNA in Translation– mRNA, rRNA, and tRNA all participate in translation.– Protein synthesis takes place on ribosomes.– A ribosome dissociates into larger and a smaller body.– In higher organisms, the larger body is called a 60S

ribosome; the smaller body is called a 40S ribosome.– Together the 40S and 60S ribosomes form a unit on which

mRNA is stretched out.– Triplets of bases on mRNA are called codonscodons..– The 20 amino acids are then brought to the mRNA-

ribosome complex, each amino acid by its own particular tRNA.

Translation• process whereby a base sequence of mRNA is used to create a protein

• the mRNA leaves the nucleus and binds with ribosomes in the cytoplasm

• transfer RNA (tRNA) contains and anticodon, which is a 3 bases sequence that is complementary to the codon on the mRNA

• tRNA also carries an amino acid

• the codon of the mRNA determines the amino acid sequence

Translation

– Activation– Initiation– Elongation– Termination

tRNA• The three-dimensional structure of tRNA.

Ribosomes

tRNA

mRNA

Protein Synthesis

• Molecular components of reactions at four stages of protein synthesis:

tRNA– Each tRNA is specific for only one amino acid.– An amino acid binds to an -OH group of the

appropriate tRNA by an ester bond.– At the opposite end of the tRNA molecule is a

codon recognition site.codon recognition site.– The codon recognition site is a sequence of three

bases called an anticodon.anticodon.– This triplet of bases aligns itself in a

complementary fashion to the codon triplet on mRNA.

Amino Acid Activation• The activated amino acid is bound to its own

particular tRNA by an ester bond between the carboxyl group of the amino acid and an -OH of the tRNA.

The Genetic Code

Features of the Code

– 64 codons– 61 code for amino acids.– 3 (UAA, UAG, and UGA) serve as termination

signals.– AUG also serves as an initiation signal.– Only Trp and Met have one codon each.– More than one triplet can code for the same amino

acid; Leu, Ser, and Arg

Features of the Code

• For the 15 amino acids coded for by 2, 3, or 4 triplets, it is only the third letter of the codon that varies. Gly, for example, is coded for by GGA, GGG, GGC, and GGU.

• The code is almost universal: it the same in viruses, prokaryotes, and eukaryotes; the only exceptions are some codons in mitochondria

Chain Elongation•The steps of chain elongation.

Chain Termination

• Chain termination requires:• Termination codons (UAA, UAG, or

UGA) of mRNA.• Releasing factors that cleave the

polypeptide chain from the last tRNA and release the tRNA from the ribosome.

Information Transfer

Mutations and Mutagens

• Mutation:Mutation: An error in the copying of a sequence of bases.– It is estimated that, on average, there is one copying error for

every 1010 bases.– Mutations can occur during replication.– Base errors can also occur during transcription in protein

synthesis (a nonheritable error).– Consider the mRNA codons for Val, which are CAT, CAC, CAG,

and CAA.– If the original codon is CAT, it may be transcribed onto mRNA as

GUC which codes for Val.– Other errors in replication may lead to a change in protein

structure and be very harmful.

Mutations and Mutagens

• Mutagen:Mutagen: a chemical that causes a base change or mutation in DNA.

• Many changes in base sequence caused by radiation and mutagens do not become mutations because cells have repair mechanisms called nucleotide excision nucleotide excision repair (NER).repair (NER).– NER can prevent mutations by cutting out damaged

areas and resynthesizing the proper sequence.• Not all mutations are harmful.

– Certain ones may be beneficial because they enhance the survival rate of the species.

1. What are the three components of every nucleotide?

2. Name two types of nucleic acids.

3. Name the base that forms a pair with each of the following bases: a) uracil b) guaninec) adenined) cytosine

4. The base sequence along one strand of DNA is A-C-T-G-T. What would be the sequence on the complementary DNA strand?

5. Describe 3 ways in which RNA differs from DNA

6. What nucleotide bases have double rings?

7. Where is DNA found?