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Nucleic Acids Structure, Function, and the Central Dogma

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Nucleic Acids

Structure, Function, and the Central Dogma

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3.7 Nucleic Acids

Some nucleotides are subunits of nucleic acids

such as DNA and RNA

Some nucleotides have roles in metabolism

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Nucleotides

Nucleotide

• A small organic molecule consisting of a sugar

with a five-carbon ring, a nitrogen-containing

base, and one or more phosphate groups

ATP

• A nucleotide with three phosphate groups

• Important in phosphate-group (energy) transfer

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ATP

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Nucleic Acids

Nucleic acids

• Polymers of nucleotides in which the sugar of one

nucleotide is attached to the phosphate group of

the next

• RNA and DNA are nucleic acids

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RNA

RNA (ribonucleic acid)

• Contains four kinds of nucleotide monomers,

including ATP

• Important in protein synthesis

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DNA

DNA (deoxyribonucleic acid)

• Two chains of nucleotides twisted together into a

double helix and held by hydrogen bonds

• Contains all inherited information necessary to

build an organism, coded in the order of

nucleotide bases

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Four Nucleotides of DNA

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Fig. 3-21, p. 48

adenine (A)

base with a

double ring

structure

thymine (T)

base with a

single ring

structure 3 phosphate

groups

sugar

(deoxyribose) guanine (G)

base with a

double ring

structure

cytosine (C)

base with a

single ring

structure

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Fig. 3-22, p. 49

covalent

bonding in

sugar–

phosphate

backbone

hydrogen bonding

between bases

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3.7 Key Concepts:

Nucleotides and Nucleic Acids

Nucleotides have major metabolic roles and are

building blocks of nucleic acids

Two kinds of nucleic acids, DNA and RNA,

interact as the cell’s system of storing, retrieving,

and translating information about building

proteins

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DNA Structure and Function

Chapter 13

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Impacts, Issues

Here Kitty, Kitty, Kitty, Kitty, Kitty

Clones made from adult cells have problems;

the cell’s DNA must be reprogrammed to

function like the DNA of an egg

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13.2 The Discovery of DNA’s Structure

Watson and Crick’s discovery of DNA’s structure

was based on almost fifty years of research by

other scientists

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DNA’s Building Blocks

Nucleotide

• A nucleic acid monomer consisting of a five-

carbon sugar (deoxyribose), three phosphate

groups, and one of four nitrogen-containing bases

DNA consists of four nucleotide building blocks

• Two pyrimidines: thymine and cytosine

• Two purines: adenine and guanine

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Four Kinds of Nucleotides in DNA

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Fig. 13-4a, p. 206

adenine (A)

deoxyadenosine triphosphate, a purine

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Fig. 13-4b, p. 206

guanine (G)

deoxyguanosine triphosphate, a purine

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Fig. 13-4c, p. 206

thymine (T)

deoxythymidine triphosphate, a pyrimidine

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Fig. 13-4d, p. 206

cytosine (C)

deoxycytidine triphosphate, a pyrimidine

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Chargaff’s Rules

The amounts of thymine and adenine in DNA

are the same, and the amounts of cytosine and

guanine are the same: A = T and G = C

The proportion of adenine and guanine differs

among species

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Franklin, Watson and Crick

Rosalind Franklin’s research in x-ray

crystallography revealed the dimensions and

shape of the DNA molecule: an alpha helix

This was the final piece of information Watson

and Crick needed to build their model of DNA

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Watson and Crick’s DNA Model

A DNA molecule consists of two nucleotide

chains (strands), running in opposite directions

and coiled into a double helix

Base pairs form on the inside of the helix, held

together by hydrogen bonds (A-T and G-C)

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Patterns of Base Pairing

Bases in DNA strands can pair in only one way

• A always pairs with T; G always pairs with C

The sequence of bases is the genetic code

• Variation in base sequences gives life diversity

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Structure of DNA

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Fig. 13-5a, p. 207

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DNA Double Helix

• Maurice Wilkins and Rosalind Franklin • James Watson and Francis Crick

Features: • two helical polynucleotides coiled around an axis • chains run in opposite directions • sugar-phosphate backbone on the outside, bases on the inside • bases nearly perpendicular to the axis • repeats every 34 Å • 10 bases per turn of the helix • diameter of the helix is 20 Å

Secondary Structure

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Double helix stabilized by hydrogen bonds.

Which is more stable?

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Axial view of DNA

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A and B forms are both right-handed double helix. A-DNA has different characteristics from the more common B-DNA.

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• left-handed • backbone phosphates zigzag

Z-DNA

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Comparison Between A, B, and Z DNA:

A-DNA: right-handed, short and broad, 11 bp per turn

B-DNA: right-handed, longer, thinner, 10 bp per turn

Z-DNA: left-handed, longest, thinnest, 12 bp per turn

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Major and minor grooves are lined with sequence-specific H-bonding.

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Supercoiling

relaxed DNA

supercoiled DNA

Tertiary Structure

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Consequences of double helical structure:

1. Facilitates accurate hereditary information transmission 2.Reversible melting

• melting: dissociation of the double helix • melting temperature (Tm) • hypochromism • annealing

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Structure of Single-stranded DNA

Stem Loop

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NUCLEIC ACIDS: STRUCTURE

RNA

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Secondary Structure

transfer RNA (tRNA) : Brings amino acids to

ribosomes during translation

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Transfer RNA

Extensive H-bonding creates four double helical domains, three capped by loops, one by a stem

Only one tRNA structure (alone) is known

Many non-canonical base pairs found in tRNA

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ribosomal RNA (rRNA) : Makes up the ribosomes, together with ribosomal proteins.

Ribosomes synthesize proteins

All ribosomes contain large and small subunits

rRNA molecules make up about 2/3 of ribosome

Secondary structure features seem to be conserved, whereas sequence is not

There must be common designs and functions that must be conserved

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messenger RNA (mRNA) : Encodes amino acid sequence of a polypeptide

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small nuclear RNA (snRNA) :With proteins, forms complexes that are used in RNA processing in eukaryotes. (Not found in prokaryotes.)

http://en.wikipedia.org/wiki/Image:RF00066.jpg
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13.2 Key Concepts

Discovery of DNA’s Structure

A DNA molecule consists of two long chains of

nucleotides coiled into a double helix

Four kinds of nucleotides make up the chains,

which are held together along their length by

hydrogen bonds

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13.3 DNA Replication and Repair

A cell copies its DNA before mitosis or meiosis I

DNA repair mechanisms and proofreading

correct most replication errors

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Semiconservative DNA Replication

Each strand of a DNA double helix is a template

for synthesis of a complementary strand of DNA

One template builds DNA continuously; the other

builds DNA discontinuously, in segments

Each new DNA molecule consist of one old

strand and one new strand

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Enzymes of DNA Replication

DNA helicase

• Breaks hydrogen bonds between DNA strands

DNA polymerase

• Joins free nucleotides into a new strand of DNA

DNA ligase

• Joins DNA segments on discontinuous strand

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DNA Replication

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Stepped Art Fig. 13-6, p. 208

D DNA ligase seals any gaps that remain

between bases of the ―new‖ DNA, so a

continuous strand forms. The base sequence

of each half-old, half-new DNA molecule is

identical to that of the parent DNA molecule.

C Each of the two parent strands serves

as a template for assembly of a new DNA

strand from free nucleotides, according to

base-pairing rules (G to C, T to A). Thus, the

two new DNA strands are complementary

in sequence to the parental strands.

B As replication starts, the two

strands of DNA are unwound. In

cells, the unwinding occurs simul-

taneously at many sites along the

length of each double helix.

A A DNA molecule is double-stranded.

The two strands of DNA stay zippered up

together because they are complementary:

their nucleotides match up according to

base-pairing rules (G to C, T to A).

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Semiconservative Replication of DNA

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Discontinuous Synthesis of DNA

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Fig. 13-8a, p. 209

A Each DNA strand has two

ends: one with a 5’ carbon,

and one with a 3’ carbon.

DNA polymerase can add

nucleotides only at the 3’

carbon. In other words, DNA

synthesis proceeds only in

the 5’ to 3’ direction.

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Fig. 13-8b, p. 209

The parent DNA

double helix

unwinds in this

direction.

Only one new

DNA strand

is assembled

continuously.

5’

The other new

DNA strand is

assembled in

many pieces.

3’

3’

Gaps are

sealed by

DNA ligase. 5’ 3’

3’ 5’

B Because DNA synthesis proceeds only in the 5’ to 3’ direction, only

one of the two new DNA strands can be assembled in a single piece.

The other new DNA strand forms in short segments, which are called

Okazaki fragments after the two scientists who discovered them. DNA

ligase joins the fragments into a continuous strand of DNA.

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DNA replication requires unwinding of the DNA helix.

expose single-stranded templates DNA gyrase – acts to overcome torsional stress imposed upon unwinding helicases – catalyze unwinding of double helix

-disrupts H-bonding of the two strands

SSB (single-stranded DNA-binding proteins) – binds to the unwound strands, preventing re-annealing

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DNA Polymerase = enzymes that replicate DNA

All DNA Polymerases share the following: 1.Incoming base selected in the active site (base-complementarity) 2.Chain growth 5’ 3’ direction (antiparallel to template)

3.Cannot initiate DNA synthesis de novo (requires primer)

First DNA Polymerase discovered – E.coli DNA Polymerase I (by Arthur Kornberg and colleagues)

Roger D. Kornberg

2006 Nobel Prize in Chemistry

Arthur Kornberg

1959 Nobel Prize in Physiology and Medicine

http://www.nobelprize.org

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Checking for Mistakes

DNA repair mechanisms

• DNA polymerases proofread DNA sequences

during DNA replication and repair damaged DNA

When proofreading and repair mechanisms fail,

an error becomes a mutation – a permanent

change in the DNA sequence

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13.3 Key Concepts

How Cells Duplicate Their DNA

Before a cell begins mitosis or meiosis, enzymes

and other proteins replicate its chromosome(s)

Newly forming DNA strands are monitored for

errors

Uncorrected errors may become mutations

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From DNA to Protein

Chapter 14

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Impacts, Issues:

Ricin and your Ribosomes

Ricin is toxic because it inactivates ribosomes,

the organelles which assemble amino acids into

proteins, critical to life processes

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14.1 DNA, RNA, and Gene Expression

What is genetic information and how does a cell

use it?

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The Nature of Genetic Information

Each strand of DNA consists of a chain of four

kinds of nucleotides: A, T, G and C

The sequence of the four bases in the strand is

the genetic information

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Converting a Gene to an RNA

Transcription

• Enzymes use the nucleotide sequence of a gene

to synthesize a complementary strand of RNA

DNA is transcribed to RNA

• Most RNA is single stranded

• RNA uses uracil in place of thymine

• RNA uses ribose in place of deoxyribose

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Ribonucleotides and Nucleotides

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Ribonucleotides and Nucleotides

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DNA and RNA

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Fig. 14-3, p. 217

DNA RNA adenine A

NH 2 deoxyribonucleic acid ribonucleic acid

adenine A

NH 2

N C N C C N nucleotide

base HC

C N HC

N N N C CH N C CH

sugar–

phosphate

backbone

guanine G

O guanine G

O

N C N C C NH C NH HC HC

N N NH 2 N NH 2

C C N C C

cytosine C NH 2

cytosine C NH 2

C C HC N HC N

HC C O HC C O N N

thymine T O base pair uracil U O

C C CH 3 C NH HC NH

HC C O HC C O N N

DNA has one function: It

permanently stores a cell’s

genetic information, which

is passed to offspring.

RNAs have various

functions. Some serve

as disposable copies of

DNA’s genetic message;

others are catalytic.

Nucleotide

bases of DNA

Nucleotide

bases of RNA

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RNA in Protein Synthesis

Messenger RNA (mRNA)

• Contains information transcribed from DNA

Ribosomal RNA (rRNA)

• Main component of ribosomes, where polypeptide

chains are built

Transfer RNA (tRNA)

• Delivers amino acids to ribosomes

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Converting mRNA to Protein

Translation

• The information carried by mRNA is decoded

into a sequence of amino acids, resulting in a

polypeptide chain that folds into a protein

mRNA is translated to protein

• rRNA and tRNA translate the sequence of base

triplets in mRNA into a sequence of amino acids

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Gene Expression

A cell’s DNA sequence (genes) contains all the

information needed to make the molecules of life

Gene expression

• A multistep process including transcription and

translation, by which genetic information encoded

by a gene is converted into a structural or

functional part of a cell or body

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14.1 Key Concepts

DNA to RNA to Protein

Proteins consist of polypeptide chains

The chains are sequences of amino acids that

correspond to sequences of nucleotide bases in

DNA called genes

The path leading from genes to proteins has two

steps: transcription and translation

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14.2 Transcription: DNA to RNA

RNA polymerase assembles RNA by linking

RNA nucleotides into a chain, in the order

dictated by the base sequence of a gene

A new RNA strand is complementary in

sequence to the DNA strand from which it was

transcribed

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DNA Replication and Transcription

DNA replication and transcription both

synthesize new molecules by base-pairing

In transcription, a strand of mRNA is assembled

on a DNA template using RNA nucleotides

• Uracil (U) nucleotides pair with A nucleotides

• RNA polymerase adds nucleotides to the

transcript

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Base-Pairing in

DNA Synthesis and Transcription

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Fig. 14-4, p. 218

Stepped Art

DNA template

New DNA strand

DNA template

RNA transcript

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Process of Transcription has four stages:

1. Binding of RNA polymerase at promoter sites 2. Initiation of polymerization 3. Chain elongation 4. Chain termination

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Transcription (RNA Synthesis)

RNA Polymerases

Template (DNA)

Activated precursors (NTP)

Divalent metal ion (Mg2+ or Mn2+)

Mechanism is similar to DNA Synthesis

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Reece R. Analysis of Genes and Genomes.2004. p47.

Limitations of RNAP II:

1. It can’t recognize its target promoter and gene. (BLIND)

2. It is unable to regulate mRNA production in response to developmental and environmental signals. (INSENSITIVE)

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Start of Transcription

Promoter Sites Where RNA Polymerase can indirectly bind

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TATA box – a DNA sequence (5’—TATAA—3’) found in the promoter region of most eukaryotic genes.

Abeles F, et al. Biochemistry. 1992. p391.

Preinitiation Complex (PIC)

Transcription Factors (TF):

Hampsey M. Molecular Genetics of RNAP. Microbiology and Molecular Biology Reviews. 1998. p7.

TFIID binds to TATA; promotes TFIIB binding

TFIIA stabilizes TBP binding

TFIIB promotes TFIIF-pol II binding

TFIIF targets pol II to promoter

TFIIE stimulates TFIIH kinase and ATPase

actiivities

TFII H helicase, ATPase, CTD kinase activities

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Termination of Transcription

Terminator Sequence

Encodes the termination signal

In E. coli – base paired hair pin (rich in GC) followed by UUU…

1. Intrinsic termination = termination sites

causes the RNAP to pause

causes the RNA strand to detach from the DNA template

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Termination of Transcription

2. Rho termination = Rho protein, ρ

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Transcription

Many RNA polymerases can transcribe a gene

at the same time

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Fig. 14-6, p. 219

RNA transcripts DNA molecule

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14.2 Key Concepts

DNA to RNA: Transcription

During transcription, one strand of a DNA double

helix is a template for assembling a single,

complementary strand of RNA (a transcript)

Each transcript is an RNA copy of a gene

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14.3 RNA and the Genetic Code

Base triplets in an mRNA are words in a protein-

building message

Two other classes of RNA (rRNA and tRNA)

translate those words into a polypeptide chain

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prokaryotes: transcription and translation happen in cytoplasm

eukaryotes: transcription (nucleus); translation (ribosome in cytoplasm)

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In eukaryotes, mRNA is modified after transcription

Capping, methylation

Poly-(A) tail

splicing

capping: guanylyl residue

capping and methylation ensure stability of the mRNA template; resistance to exonuclease activity

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Post-Transcriptional Modifications

In eukaryotes, RNA is modified before it leaves

the nucleus as a mature mRNA

Introns

• Nucleotide sequences that are removed from a

new RNA

Exons

• Sequences that stay in the RNA

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Alternative Splicing

Alternative splicing

• Allows one gene to encode different proteins

• Some exons are removed from RNA and others

are spliced together in various combinations

After splicing, transcripts are finished with a

modified guanine “cap” at the 5' end and a poly-

A tail at the 3' end

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Post-Transcriptional Modifications

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Fig. 14-7, p. 220

gene

exon intron exon intron exon

DNA

transcription into RNA

cap poly-A tail

RNA 5’ 3’

snipped out snipped out

mRNA

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gene

exon intron exon intron exon

DNA

Fig. 14-7, p. 220

Stepped Art

transcription into RNA

cap poly-A tail

RNA 5’ 3’

snipped out snipped out

mRNA

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mRNA – The Messenger

mRNA carries protein-building information to

ribosomes and tRNA for translation

Codon

• A sequence of three mRNA nucleotides that

codes for a specific amino acid

• The order of codons in mRNA determines the

order of amino acids in a polypeptide chain

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Properties of mRNA

1. In translation, mRNA is read in groups of bases called “codons”

2. One codon is made up of 3 nucleotides from 5’ to 3’ of mRNA

3. There are 64 possible codons

4. Each codon stands for a specific amino acid, corresponding to the genetic code

5. However, one amino acid has many possible codons. This property is termed degeneracy

6. 3 of the 64 codons are terminator codons, which signal the end of translation

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Genetic Information

From DNA to mRNA to amino acid sequence

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Genetic Code

Genetic code

• Consists of 64 mRNA codons (triplets)

• Some amino acids can be coded by more than

one codon

Some codons signal the start or end of a gene

• AUG (methionine) is a start codon

• UAA, UAG, and UGA are stop codons

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Codons of the Genetic Code

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Encoded sequences.

(a) Write the sequence of the mRNA molecule synthesized from a DNA template strand having the sequence

(b) What amino acid sequence is encoded by the following base sequence of an mRNA molecule? Assume that the reading frame starts at the 5 end.

Practice

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Answers

(a) 5’ -UAACGGUACGAU-3’ .

(b) Met-Pro-Ser-Asp-Trp-Met.

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rRNA and tRNA – The Translators

tRNAs deliver amino acids to ribosomes

• tRNA has an anticodon complementary to an

mRNA codon, and a binding site for the amino

acid specified by that codon

Ribosomes, which link amino acids into

polypeptide chains, consist of two subunits of

rRNA and proteins

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Ribosomes

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tRNA

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14.3 Key Concepts

RNA

Messenger RNA carries DNA’s protein-building instructions

Its nucleotide sequence is read three bases at a time

Sixty-four mRNA base triplets—codons—represent the genetic code

Two other types of RNA interact with mRNA during translation of that code

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14.4 Translation: RNA to Protein

Translation converts genetic information carried

by an mRNA into a new polypeptide chain

The order of the codons in the mRNA

determines the order of the amino acids in the

polypeptide chain

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Translation

Translation occurs in the cytoplasm of cells

Translation occurs in three stages

• Initiation

• Elongation

• Termination

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Initiation

An initiation complex is formed

• A small ribosomal subunit binds to mRNA

• The anticodon of initiator tRNA base-pairs with

the start codon (AUG) of mRNA

• A large ribosomal subunit joins the small

ribosomal subunit

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Elongation

The ribosome assembles a polypeptide chain as

it moves along the mRNA

• Initiator tRNA carries methionine, the first amino

acid of the chain

• The ribosome joins each amino acid to the

polypeptide chain with a peptide bond

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Termination

When the ribosome encounters a stop codon,

polypeptide synthesis ends

• Release factors bind to the ribosome

• Enzymes detach the mRNA and polypeptide

chain from the ribosome

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Polysomes

Many ribosomes may

simultaneously

translate the same

mRNA, forming

polysomes

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Initiation

A A mature mRNA

leaves the nucleus and

enters cytoplasm, which

has many free amino

acids, tRNAs, and

ribosomal subunits. initiator

tRNA

small

ribosomal

subunit

mRNA

Fig. 14-12 (a-b), p. 222

Stepped Art

An initiator tRNA binds

to a small ribosomal

subunit and the

mRNA.

large

ribosomal

subunit

B A large ribosomal

subunit joins, and

the cluster is now

called an initiation

complex.

Translation in

Eukaryotes

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Translation in Eukaryotes

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Fig. 14-12c, p. 223

Elongation

C An initiator tRNA

carries the amino acid

methionine, so the first

amino acid of the new

polypeptide chain will be

methionine. A second

tRNA binds the second

codon of the mRNA (here,

that codon is GUG, so the

tRNA that binds carries

the amino acid valine).

A peptide bond

forms between

the first two

amino acids

(here, methionine

and valine).

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Fig. 14-12d, p. 223

D The first tRNA is

released and the

ribosome moves to the

next codon in the mRNA.

A third tRNA binds to the

third codon of the mRNA

(here, that codon is UUA,

so the tRNA carries the

amino acid leucine).

A peptide bond

forms between the

second and third

amino acids

(here, valine

and leucine).

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Fig. 14-12e, p. 223

E The second tRNA

is released and the

ribosome moves to the

next codon. A fourth

tRNA binds the fourth

mRNA codon (here, that

codon is GGG, so the

tRNA carries the amino

acid glycine).

A peptide bond

forms between the

third and fourth

amino acids (here,

leucine and

glycine).

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Fig. 14-12f, p. 223

Termination

F Steps d and e are repeated over and

over until the ribosome encounters a stop

codon in the mRNA. The mRNA transcript

and the new polypeptide chain are

released from the ribosome. The two

ribosomal subunits separate from each

other. Translation is now complete. Either

the chain will join the pool of proteins in

the cytoplasm or it will enter rough ER of

the endomembrane system (Section 4.9).

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14.4 Key Concepts

RNA to Protein: Translation

Translation is an energy-intensive process by

which a sequence of codons in mRNA is

converted to a sequence of amino acids in a

polypeptide chain

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14.5 Mutated Genes

and Their Protein Products

If the nucleotide sequence of a gene changes, it

may result in an altered gene product, with

harmful effects

Mutations

• Small-scale changes in the nucleotide sequence

of a cell’s DNA that alter the genetic code

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Common Mutations

Base-pair-substitution

• May result in a premature stop codon or a

different amino acid in a protein product

• Example: sickle-cell anemia

Deletion or insertion

• Can cause the reading frame of mRNA codons to

shift, changing the genetic message

• Example: Huntington’s disease

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Common Mutations

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Fig. 14-13, p. 224

part of DNA

A Part of the DNA,

mRNA, and amino acid

sequence of the beta

chain of a normal

hemoglobin molecule.

mRNA transcribed

from DNA

resulting amino acid sequence

THREONINE PROLINE GLUTAMATE GLUTAMATE LYSINE

base substitution

in DNA

B A base-pair

substitution in DNA

replaces a thymine

with an adenine. When

the altered mRNA is

translated, valine

replaces glutamate as

the sixth amino acid of

the new polypeptide

chain. Hemoglobin with

this chain is HbS—sickle

hemoglobin (Section

3.6).

altered mRNA

altered amino acid sequence

THREONINE PROLINE VALINE GLUTAMATE LYSINE

deletion in DNA

C Deletion of the

same thymine causes a

frameshift. The reading

frame for the rest of the

mRNA shifts, and a

different protein product

forms. This mutation

results in a defective

hemoglobin molecule. The

outcome is thalassemia, a

type of anemia.

altered mRNA

altered amino acid sequence

THREONINE PROLINE GLYCINE ARGININE

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What Causes Mutations?

Transposable elements

• Segments of DNA that can insert themselves

anywhere in a chromosomes

Spontaneous mutations

• Uncorrected errors in DNA replication

Harmful environmental agents

• Ionizing radiation, UV radiation, chemicals

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Agents of Mutations 1. Physical Agents

a) UV Light b) Ionizing Radiation

2. Chemical Agents Some chemical agents can be

classified further into a) Alkylating b) Intercalating c) Deaminating

3. Viral

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UV Light Causes Pyrimidine Dimerization

Replication and gene expression are blocked

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Mutations Caused by Radiation

Ionizing radiation damages chromosomes,

nonionizing (UV) radiation forms thymine dimers

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Chemical mutagens

• 5-bromouracil and 2-aminopurine can be

incorporated into DNA

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Deaminating agents

Ex: Nitrous acid (HNO2) Converts adenine to hypoxanthine, cytosine to uracil, and guanine

to xanthine

Causes A-T to G-C transitions

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Alkylating agents

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Intercalating agents

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Acridines Intercalate in DNA, leading to insertion or deletion

The reading frame during translation is changed

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DNA Repair

Direct repair Photolyase cleave pyrimidine dimers

Base excision repair E. coli enzyme AlkA removes modified bases such as 3-

methyladenine (glycosylase activity is present)

Nucleotide excision repair Excision of pyrimidine dimers (need different enzymes

for detection, excision, and repair synthesis)

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Inherited Mutations

Mutations in somatic cells of sexually

reproducing species are not inherited

Mutations in a germ cell or gamete may be

inherited, with evolutionary consequences

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14.5 Key Concepts

Mutations

Small-scale, permanent changes in the

nucleotide sequence of DNA may result from

replication errors, the activity of transposable

elements, or exposure to environmental hazards

Such mutation can change a gene’s product

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Summary:

Protein Synthesis in Eukaryotic Cells

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Fig. 14-16, p. 226

Transcription Assembly of RNA on unwound

regions of DNA molecule

mRNA rRNA tRNA mRNA

processing

proteins

mature mRNA transcripts

ribosomal subunits

mature tRNA

Convergence

of RNAs Translation cytoplasmic

pools of

amino

acids,

ribosomal

subunits, and

tRNAs

At an intact

ribosome,

synthesis of a

polypeptide

chain at the

binding sites

for mRNA and

tRNAs

Protein

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13.4 Using DNA to

Duplicate Existing Mammals

Reproductive cloning is a reproductive

intervention that results in an exact genetic copy

of an adult individual

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Cloning

Clones

• Exact copies of a molecule, cell, or individual

• Occur in nature by asexual reproduction or

embryo splitting (identical twins)

Reproductive cloning technologies produce an

exact copy (clone) of an individual

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Reproductive Cloning Technologies

Somatic cell nuclear transfer (SCNT)

• Nuclear DNA of an adult is transferred to an

enucleated egg

• Egg cytoplasm reprograms differentiated (adult)

DNA to act like undifferentiated (egg) DNA

• The hybrid cell develops into an embryo that is

genetically identical to the donor individual

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Somatic Cell Nuclear

Transfer (SCNT)

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Fig. 13-9a, p. 210

A A cow egg is held in place by

suction through a hollow glass

tube called a micropipette. The

polar body (Section 10.5) and

chromosomes are identified by

a purple stain.

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Fig. 13-9b, p. 210

B A micropipette punctures the

egg and sucks out the polar body

and all of the chromosomes. All

that remains inside the egg’s

plasma membrane is cytoplasm.

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Fig. 13-9c, p. 210

C A new micropipette prepares to

enter the egg at the puncture site.

The pipette contains a cell grown

from the skin of a donor animal.

skin cell

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Fig. 13-9d, p. 210

D The micropipette enters the

egg and delivers the skin cell to

a region between the cytoplasm

and the plasma membrane.

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Fig. 13-9e, p. 210

E After the pipette is withdrawn,

the donor’s skin cell is visible next

to the cytoplasm of the egg. The

transfer is complete.

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Fig. 13-9f, p. 210

F The egg is exposed to an electric

current. This treatment causes the

foreign cell to fuse with and empty

its nucleus into the cytoplasm of the

egg. The egg begins to divide, and an

embryo forms. After a few days, the

embryo may be transplanted into a

surrogate mother.

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A Clone Produced by SCNT

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Therapeutic Cloning

Therapeutic cloning uses SCNT to produce

human embryos for research purposes

Researchers harvest undifferentiated (stem)

cells from the cloned human embryos

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13.4 Key Concepts

Cloning Animals

Knowledge about the structure and function of

DNA is the basis of several methods of making

clones, which are identical copies of organisms

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13.5 Fame and Glory

In science, as in other professions, public

recognition does not always include everyone

who contributed to a discovery

Rosalind Franklin was first to discover the

molecular structure of DNA, but did not share in

the Nobel prize which was given to Watson,

Crick, and Wilkins

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Rosalind Franklin’s

X-Ray Diffraction Image

Franklin died of cancer at age 37, possibly

related to extensive exposure to x-rays

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13.5 Key Concepts

The Franklin Footnote

Science proceeds as a joint effort; many

scientists contributed to the discovery of DNA’s

structure


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Nucleotides and nucleic acids - - Get a Free

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