Molecular Biology of the Gene

Molecular biology

is the study of heredity at the molecular level.

nucleotides

DNA and RNA are nucleic acids consisting of long chains (polymers) of chemical units (monomers) called…

A nucleotide is composed of a 

• nitrogenous base,

• five-carbon sugar, and

• phosphate group. 

 DNA polynucleotide

One of the two strands of DNA is a….

 sugar-phosphate backbone

Nucleotides are joined to one another by a…

DNA nucleotide has a different nitrogen-containing base:

• adenine (A),

• cytosine (C),

• thymine (T), and

• guanine (G).

RNA (ribonucleic acid)

• uses the sugar ribose (instead of deoxyribose in DNA) and 

• has a nitrogenous base uracil (U) instead of thymine.

Watson

deduced the basic shape of DNA to be a helix (spiral) with a uniform diameter and the nitrogenous bases located above one another like a stack of dinner plates.

James D. Watson, Francis Crick, and Maurice Wilkins.

 1962, the Nobel Prize was awarded to…

Rosalind Franklin 

probably would have received the prize as well but for her death from cancer in 1958. 

The Watson-Crick model

 gave new meaning to the words genes and chromosomes.

semiconservative model

DNA replication follows a….

Each new DNA helix has…

one old strand with one new strand

 origins of replication

Replication of a DNA molecule begins at particular sites called…

Proteins that initiate DNA replication

• attach to the DNA at the origin of replication and

• separate the two strands of the double helix.

“bubbles.” 

Replication then proceeds in both directions, creating replication…

 5′ to 3′ direction

DNA replication occurs in the…

• 3′ to 5′ template

• Replication is continuous on the…

• DNA polymerases 

• add nucleotides only to the 3′ end of the strand, never to the 5′ end. 

• 5′ to 3′ template

• Replication is discontinuous on the…, forming short Okazaki fragments.

• DNA ligase,

• An enzyme, called … links (or ligates) the pieces together into a single DNA strand. 

DNA damaged by harmful radiation and toxic chemicals

DNA polymerases and DNA ligase also repair…

genotype and phenotype. 

Proteins are the links between…

Genes 

 provide the instructions for making specific proteins

Transcription 

is the synthesis of RNA under the direction of DNA

Translation

is the synthesis of proteins under the direction of RNA.

Provides a code for constructing a protein

The sequence of nucleotides in DNA …

“language.”

• Transcription rewrites the DNA code into RNA, using the same nucleotide…

triplet code

The flow of information from gene to protein is based on a…

codons

The genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of nonoverlapping three-base “words” called…

codon

Each amino acid is specified by a …

64 codons

 …. are possible.

one possible codon

Some amino acids have more than….

genetic code

The… is the amino acid translations of each of the nucleotide triplets.

Three nucleotides

… specify one amino acid.

AUG codes

 for methionine and signals the start of transcription.

Three “stop” codons

signal the end of translation (UAA, UAG, UGA).

redundant

The genetic code is

• … with more than one codon for some amino acids,

unambiguous

The genetic code is … in that any codon for one amino acid does not code for any other amino acid

nearly universal

The genetic code is … in that the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals.

Transcription initiation

involving the attachment of RNA
polymerase to the promoter and the start of RNA
synthesis,

Transcription elongation

as the newly formed RNA strand
grows

Transcription termination

when RNA polymerase reaches the
terminator DNA and the polymerase molecule
detaches from the newly made RNA strand and
the gene.

Messenger RNA (mRNA)

• encodes amino acid sequences and

• conveys genetic messages from DNA to the translation machinery of the cell. 

same place

In prokaryotes, this occurs in the…that mRNA is made. 

exit

 in eukaryotes, mRNA must … the nucleus via nuclear pores to enter the cytoplasm.

introns, exons

Eukaryotic mRNA has … interrupting sequences that separate … the coding regions.

RNA splicing 

removes introns (intervening sequences) and joins exons (expressed sequences) to produce a continuous coding sequence

A cap and tail of extra nucleotides are added to the ends of the mRNA to

• facilitate the export of the mRNA from the nucleus, 

• protect the mRNA from degradation by cellular enzymes, and 

• help ribosomes bind to the mRNA.

not translated into protein.

The cap and tail themselves are …

Transfer RNA (tRNA)

 molecules function as an interpreter, converting the genetic message of mRNA into the language of proteins.

anticodon

Transfer RNA molecules perform this interpreter task by

picking up the appropriate amino acid and

using a special triplet of bases, called an … , to recognize the appropriate codons in the mRNA.

 ribosome

Translation occurs on the surface of the …

ribosomal RNAs

• Each subunit is composed of …and proteins.

• mRNA and tRNAs.

• Ribosomes have binding sites for…

inactivate bacterial ribosomes

Certain antibiotic drugs can…while leaving eukaryotic ribosomes unaffected.

These drugs, such as tetracycline and streptomycin,

….are used to combat bacterial infections

Translation

1. initiation,

2. elongation, and

3. termination.

• mRNA,

• a tRNA bearing the first amino acid, and

• the two subunits of a ribosome.

 Initiation brings together…

start codon.

1. An mRNA molecule binds to a small ribosomal subunit, and a special initiator tRNA binds to mRNA at the…

start codon

• The…reads AUG and codes for methionine.

anticodon

The first tRNA has the … UAC.

large ribosomal subunit

A …joins the small subunit, allowing the ribosome to function.

P site

The first tRNA occupies the… which will hold the growing polypeptide.

A site

The … is available to receive the next amino-acid-bearing tRNA.

Elongation

 adds amino acids to the polypeptide chain until a stop codon terminates translation

mRNA codon in the A site of the ribosome.

The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the …

a new peptide bond to the amino acid carried by the tRNA in the A site.

The polypeptide separates from the tRNA in the P site and attaches by….

1. tRNA (which has the growing polypeptide) from the A site to the P site. 

1. The P site tRNA (now lacking an amino acid) leaves the ribosome, and the ribosome translocates (moves) the remaining …

stop codon,

Elongation continues until the termination stage of translation, when

• the ribosome reaches a…

 DNA to RNA to protein

The flow of genetic information is from …

 in the nucleus, and the messenger RNA is processed before it travels to the cytoplasm.

In eukaryotic cells, transcription occurs….

in the cytoplasm

In prokaryotes, transcription occurs…

Translation can be divided into four steps, all of which occur in the cytoplasm:

1. amino acid attachment,

2. initiation of polypeptide synthesis,

3. elongation, and

4. termination.

mutation

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

Mutations can involve …

large chromosomal regions or 

just a single nucleotide pair.

Base substitutions

…. may

have no effect at all

nonsense mutation

change an amino acid into a stop codon, producing a…

 frameshift mutation

Nucleotide insertions or deletions of one or more nucleotides in a gene may

cause a… which alters the reading frame (triplet grouping) of the genetic message,lead to significant changes in amino acid sequence, and

produce a nonfunctional polypeptide.

Causes of frameshift mutations:

A deletion from DNA 

Mutation in which one or more base pairs are lost.

An insertion mutation can also alter polypeptides.

Mutation in which one or more base pairs are added.

deletion

A … from DNA 

Mutation in which one or more base pairs are lost.

insertion

An ….mutation can also alter polypeptides.

Mutation in which one or more base pairs are added.

base-pair-substitution,

In… a nucleotide and its partner are replaced by a different base pair.

base-pair substitution

A … is a type of mutation in which a single base-pair changes.

Base pair substitution

 the replacement of one base by another

substitution of valine for glutamic acid

Sickle-cell disease results from a…

Mutagenesis

… is the production of mutations.

Mutations can be caused

spontaneous errors that occur during DNA replication or recombination or

by mutagens, which include high-energy radiation such as X-rays and ultraviolet light and chemicals.

polymers of nucleotides

DNA and RNA are…