BIOL302- Chapter 5: DNA and Chromosomes

What is the main function of DNA?

DNA stores hereditary genetic information that can be passed from cell to cell and generation to generation.

What is a gene?

A region of DNA that contains information used to produce a functional product, usually a protein or RNA.

What did Watson and Crick determine in 1953?

They determined the structure of DNA and showed how its structure could explain copying and information storage.

What did Rosalind Franklin contribute to the discovery of DNA structure?

She obtained important X-ray diffraction patterns of B-form DNA that revealed key structural features.

What did Fred Griffith discover?

He discovered that a substance from heat-killed pathogenic S bacteria could permanently transform harmless R bacteria into pathogenic S bacteria.

What are the two forms of Streptococcus pneumoniae used in Griffith's experiment?

S strain is smooth and pathogenic; R strain is rough and harmless.

What did Avery, MacLeod, and McCarty demonstrate?

They showed that DNA, rather than protein, was the substance capable of transforming R bacteria into pathogenic S bacteria.

What did Hershey and Chase demonstrate?

They showed that DNA is the genetic material of viruses.

Why was 32P used to label DNA in the Hershey-Chase experiment?

DNA contains phosphorus, while protein does not contain phosphorus as a major component.

Why was 35S used to label protein in the Hershey-Chase experiment?

Proteins can contain sulfur, while DNA does not contain sulfur.

What happened to most of the 32P-labeled DNA in the Hershey-Chase experiment?

Most of it entered the bacterial cells.

What happened to most of the 35S-labeled protein?

Most remained outside the bacterial cells in solution.

What is chromatin?

The combination of DNA and proteins that make up the contents of the nucleus.

When do chromosomes become visible under a light microscope?

When they condense in preparation for cell division.

What are the four bases in DNA?

Adenine (A), cytosine (C), guanine (G), and thymine (T).

What is a DNA nucleotide made of?

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

How are nucleotides connected within a DNA strand?

They are covalently linked to form a sugar-phosphate backbone.

How many polynucleotide strands make up DNA?

Two.

What holds the two DNA strands together?

Hydrogen bonds between complementary bases.

What does antiparallel mean in DNA?

The two DNA strands run in opposite directions: one 5'→3' and the other 3'→5'.

Which bases pair together in DNA?

A pairs with T, and G pairs with C.

How many hydrogen bonds are between A and T?

Two hydrogen bonds.

How many hydrogen bonds are between G and C?

Three hydrogen bonds.

What is a purine?

A two-ring nitrogenous base; adenine and guanine are purines.

What is a pyrimidine?

A single-ring nitrogenous base; cytosine and thymine are pyrimidines.

Why is each base pair made of one purine and one pyrimidine?

This maintains a consistent width of the DNA double helix.

What is found at the 3' end of a DNA strand?

A free hydroxyl (-OH) group.

What is found at the 5' end of a DNA strand?

A free phosphate group.

What shape does DNA form?

A right-handed double helix.

How many bases are approximately found per turn of the DNA helix?

10 bases per turn.

What are the two grooves of DNA?

The major groove and minor groove.

Which DNA groove is wider?

The major groove.

What does a protein-coding gene produce first?

RNA.

What does the RNA produced from a protein-coding gene direct?

The production of a specific protein.

What is chromosome painting?

A technique using fluorescent DNA probes to identify individual chromosomes.

What is a karyotype?

Chromosomes arranged in homologous pairs for identification and analysis.

What are homologous chromosomes?

Chromosomes that form matching pairs and contain corresponding genes.

What is a reciprocal chromosomal translocation?

A segment of one chromosome is swapped with a segment of another chromosome.

Why are chromosomal translocations important?

They can cause genetic abnormalities and are frequent in cancer cells.

What is noncoding DNA?

DNA sequences that do not encode proteins.

Is noncoding DNA necessarily useless?

No. Noncoding DNA can have regulatory and structural functions.

What are some functions of noncoding DNA?

Regulation of protein-coding sequences, scaffold attachment, replication origins, centromeres, and telomeres.

What happens during interphase?

The cell expresses genes and replicates its DNA.

What is mitosis?

Division of the nucleus.

What happens to chromosomes during M phase?

They condense, attach to the mitotic spindle, and are separated into daughter cells.

What three DNA sequence elements are required for chromosome replication and segregation?

Origins of replication, centromeres, and telomeres.

What is the function of an origin of replication?

It is where DNA replication begins.

What is the function of the centromere?

It helps hold duplicated chromosomes together and attaches them to the mitotic spindle for proper segregation.

What is the function of telomeres?

They protect chromosome ends and help allow the ends of chromosomes to be replicated.

What are telomeres made of?

Repeated nucleotide sequences at the ends of chromosomes.

What is the nuclear envelope?

The membrane surrounding the nucleus that contains nuclear pores.

What is the nuclear lamina?

A network of protein filaments supporting the inner nuclear membrane.

What is heterochromatin?

Highly condensed, gene-poor chromatin.

Where is heterochromatin mainly located in the nucleus?

Near the nuclear periphery, underneath the nuclear envelope.

What is the nucleolus?

A nuclear region containing genes for ribosomal RNA; these genes are clustered there.

What are the two major classes of proteins associated with eukaryotic chromosomes?

Histones and nonhistone chromosome proteins.

What is a nucleosome?

DNA wrapped around a core of histone proteins.

How many histone proteins are in a nucleosome core?

Eight histones: two each of H2A, H2B, H3, and H4.

How much DNA is wrapped around the histone core of a nucleosome?

About 147 nucleotide pairs.

How many turns does the DNA make around the histone core?

About 1.7 turns.

Why do histones bind tightly to DNA?

Histones contain positively charged amino acids that interact with negatively charged DNA.

Which amino acids give histones their positive charge?

Lysine and arginine.

What is histone H1?

A linker histone that helps pull nucleosomes together and contributes to formation of the 30-nm fiber.

What do cohesin and condensin proteins help with?

They help organize and compact chromatin.

What are looped chromatin domains?

Regions where chromatin is folded into loops anchored by nonhistone chromosomal proteins.

What do condensins do to chromatin?

They form loops within loops and help fold chromosomes into a highly compact configuration.

What is euchromatin?

Less condensed chromatin that is generally more accessible for gene expression.

What is heterochromatin compared with euchromatin?

Heterochromatin is more condensed than euchromatin.

What is constitutive heterochromatin?

A region of DNA that is permanently condensed.

What is facultative heterochromatin?

A region that can switch between open and compact states.

What is quiescent euchromatin?

Less-condensed euchromatin that is currently inactive or has low transcriptional activity.

What do chromatin-remodeling complexes do?

They use ATP hydrolysis to reposition nucleosomes and make DNA more accessible.

How can chromatin-remodeling complexes loosen chromatin?

They push DNA along the histone core, repositioning nucleosomes and exposing DNA.

What types of chemical groups can modify histone tails?

Acetyl, methyl, and phosphate groups.

Where do most histone modifications occur?

On the N-terminal tails of histones.

What can different combinations of histone tail modifications do?

They can determine how a region of chromatin is treated by the cell.

How can heterochromatin spread?

Repressive histone modifications attract proteins that reproduce the modifications on neighboring nucleosomes.

What prevents heterochromatin from spreading indefinitely?

DNA barrier sequences.

How can heterochromatin patterns be inherited after DNA replication?

Parent H3 and H4 histones are passed to daughter DNA with their modifications, which help reestablish the same chromatin pattern.

What is a position effect?

When a gene's activity changes because its position in the chromosome changes.

What happens to the White gene when it is moved closer to heterochromatin in fruit flies?

It can become silenced in some cells, producing mottled red-and-white eyes.

Why can moving a gene change its expression?

Its new location may place it near chromatin regions that promote or repress gene activity.

What is X chromosome inactivation?

The process in which one X chromosome in each female mammalian cell becomes condensed into heterochromatin and largely inactive.

Is the X chromosome that becomes inactive chosen randomly?

Yes, it is apparently random in each early embryonic cell.

Why are female mammals mosaics for X chromosome activity?

Different cells randomly inactivate either the maternal or paternal X chromosome, and that choice is maintained in their descendants.