Nucleic Acids

What are the three components of a nucleotide?

A pentose sugar, a nitrogenous base, and a phosphate group.

What type of bond joins two nucleotides together?

A phosphodiester bond.

What reaction forms a phosphodiester bond, and what reaction breaks it?

Condensation forms it; hydrolysis breaks it.

What is the "sugar-phosphate backbone"?

The repeating chain of sugars and phosphates in a polynucleotide, held by covalent bonds.

Name the pentose sugar found in DNA.

Deoxyribose.

Name the four nitrogenous bases in DNA.

Adenine (A), Thymine (T), Guanine (G), and Cytosine (C).

Define a purine and name the two found in DNA.

A larger base with a double-carbon ring structure; Adenine and Guanine.

Define a pyrimidine and name the two found in DNA.

A smaller base with a single-carbon ring structure; Cytosine and Thymine.

Why is the constant distance between DNA strands maintained?

Because a large purine always pairs with a small pyrimidine.

How many hydrogen bonds form between A and T?

Two.

How many hydrogen bonds form between C and G?

Three.

What does antiparallel mean in DNA structure?

The two strands run in opposite directions (one 5' to 3', the other 3' to 5').

List three structural differences between DNA and RNA.

1. DNA has deoxyribose, RNA has ribose. 2. DNA has thymine, RNA has uracil. 3. DNA is double-stranded, RNA is single-stranded.

Why is DNA described as a double helix?

Two polynucleotide strands wound around each other.

What is the function of the sugar-phosphate backbone?

It provides structural support and protects the chemically reactive organic bases inside.

Why is DNA replication called semi-conservative?

Each new DNA molecule consists of one original strand and one newly synthesized strand.

What is the role of DNA helicase?

It breaks the hydrogen bonds between complementary bases to unzip the double helix.

What is the role of DNA polymerase?

It joins free nucleotides together via condensation reactions to form the new sugar-phosphate backbone.

In which direction does DNA polymerase work?

The 5' to 3' direction.

Why are hydrogen bonds described as weak in DNA?

They are strong enough to hold strands together but weak enough to be broken during replication and transcription.

What provides the template for replication?

Both original DNA strands.

Define a gene.

A short section of DNA that codes for a specific polypeptide.

What is a locus?

The fixed position of a gene on a chromosome.

What are histones?

Proteins that DNA wraps around to become tightly packed.

Contrast DNA location in prokaryotes and eukaryotes.

Prokaryotes: cytoplasm. Eukaryotes: nucleus.

Contrast DNA shape in prokaryotes and eukaryotes.

Prokaryotes: short and circular. Eukaryotes: long and linear.

Do prokaryotes have histones?

No.

Define introns.

Non-coding regions of DNA within a gene.

Define exons.

Coding regions of DNA translated into amino acids.

What is a genome?

The complete set of genes in a cell.

What is a proteome?

The full range of proteins a cell can produce.

What is a triplet?

A sequence of three DNA bases that codes for one amino acid.

What does it mean that the genetic code is universal?

The same triplets code for the same amino acids in almost all organisms.

What does non-overlapping mean in genetics?

Each base is read only once as part of one triplet.

What does degenerate mean in genetics?

More than one triplet can code for the same amino acid.

What is a codon?

A sequence of three bases on mRNA that codes for an amino acid.

What is an anticodon?

A sequence of three bases on tRNA complementary to an mRNA codon.

Where does transcription occur in eukaryotes?

In the nucleus.

Which enzyme synthesizes mRNA?

RNA polymerase.

Which DNA strand acts as the template during transcription?

The antisense strand.

How is the mRNA sequence related to the DNA sense strand?

They are the same except mRNA has uracil instead of thymine.

What is pre-mRNA?

The immediate product of transcription containing introns and exons.

Define splicing.

The removal of introns and joining of exons to form mature mRNA.

Why does splicing not occur in prokaryotes?

Because prokaryotic DNA does not contain introns.

How does mRNA leave the nucleus?

Through nuclear pores.

Where does translation take place?

On a ribosome in the cytoplasm.

What is the start codon?

AUG.

What is the role of tRNA in translation?

To carry specific amino acids to the ribosome.

What shape does tRNA have?

A clover-leaf shape held by hydrogen bonds.

What type of bond forms between amino acids?

A peptide bond.

What molecule provides energy for peptide bond formation?

ATP.

When does translation stop?

When the ribosome reaches a stop codon.

How many tRNA molecules can fit inside a ribosome at once?

Two.

Define a gene mutation.

A change in the base sequence of DNA.

What are mutagens?

External factors that increase mutation rates, such as UV light, radiation, and chemicals.

What is a substitution mutation?

One base is replaced by another base.

What is a deletion mutation?

One base is removed from the DNA sequence.

What is an insertion mutation?

One base is added to the DNA sequence.

Why are deletion and insertion mutations usually more damaging than substitutions?

They cause a frameshift affecting all downstream codons.

What is a silent mutation?

A substitution that does not change the amino acid produced.

How can a mutation affect enzyme function?

It changes the amino acid sequence and alters the active site's shape.

What is polyploidy?

Having more than two complete sets of chromosomes.

What is non-disjunction?

Failure of homologous chromosomes or sister chromatids to separate during meiosis.

What does non-disjunction result in?

Gametes or offspring with extra or missing chromosomes.

Contrast mRNA and tRNA in shape.

mRNA is linear; tRNA is clover-leaf shaped.

Contrast mRNA and tRNA in size.

mRNA is longer and variable; tRNA is short and fixed in size.

Does mRNA contain hydrogen bonds?

No.

Does tRNA contain hydrogen bonds?

Yes.

What is the difference between the antisense and sense strand?

The antisense strand is the template; the sense strand matches the mRNA sequence.

Why is DNA called the instruction manual for proteins?

The base sequence determines amino acid sequence, protein shape, and protein function.