Biology 302- Chapter 5: DNA and Chromosomes

Who were James Watson & Francis Crick?

They were the men who helped discover the structure of DNA. They additionally revealed how DNa might be copied and encodes the instructions for making proteins.

Who was Rosalind Franklin?

She was a X-ray crystallographer who obtained excellent diffraction patterns of B-form DNA and discovered important basic facts about its structure.

How did scientists discover that DNA was the genetic material?

Fred Griffith performed an experiment using two strains of pneumococcal bacterium, a pathogenic s-strain and a non-lethal r-strain. When the living r-strain and the heat-killed s-strain were both injected in the mice, the mouse still died. Then the strain cells were isolated to find what caused the mouse to die. It was determined that the molecule that carries the heritable “transforming principle” is DNA.

What is chromatin?

It is the combination of DNA and proteins that make up the contents of the nucleus of a cell.

What are purines? What nucleotides do they consist of?

Purines are organic compounds with double ring structures. The purine nucleotides are Adenine and Guanine.

What is this an image of?

Purines, specifically guanine and adenine.

What are pyrimidines? What nucleotides do they consist of?

Pyrimidines are organic compounds with single ring structures. The pyrimidine nucleotides are Cytosine, Thymine, and Uracil.

What is this an image of?

Pyrimidines, specifically thymine, cytosine, and uracil.

What are the nucleotides in DNA that bond together? RNA?

G forms a triple hydrogen bond with C, A forms a double hydrogen bond with T.

What is the bond that makes up the sugar-phosphate backbone of DNA? What type of bond is it?

It is the phosphodiester bond; covalent.

What is the difference between a nucleotide and a nucleoside?

\-tide = sugar-phosphate covalently linked to a base

\-side = sugar covalently linked to a base (no phosphate group)

Identify the 5’ and 3’ ends on this nucleotide.

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Bonus: what is this nucleotide’s base?

DNA strands run ______ to each other in the DNA molecule.

anti-parallel

\[T/F\] Each groove in the structure of DNA is the same size.

False. There is a major groove and a minor groove.

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The image on the “term” side is over-simplified and neglects to show the difference in grooves.

What is the name of the process of nuclear division?

Mitosis

What is this an image of?

Homologous chromosomes that are numbered and arranged into pairs. This is called a **karyotype**.

\[T/F\] Abnormal chromosomes have little to no effect on our genome.

False, they are associated with some inherited genetic defects.

\[T/F\] Reciprocal chromosomal translocation is beneficial, as it ensures genetic diversity.

False. When reciprocal chromosomal translocation occurs, it causes an uncommon arrangement of chromosomes. It is typically associated with cancerous cells.

\[T/F\] Despite being called “junk DNA”, non-coding DNA actually serves a purpose.

True, though we do not have enough studies that show what the full range of their purpose is yet.

What is non-coding DNA?

Non-coding DNA are sequences of DNA that do not encode protein. These sequences of DNA are __not__ biologically inactive, as the serve other purposes.

What is interphase?

It is the stage where a cell is preparing for cell division.

What is mitosis?

The division of the nucleus within the cell after the DNA has been replicated/doubled.

What occurs during the M-phase of a cell?

The chromosomes condense, gene expression largely ceases, and the mitotic spindle forms from microtubules and other proteins.

Identify numbers 1, 2, and 3 on the image.

1: telomere

2: replication origin

3: centromere

The _______ is attaches the duplicated chromosomes to the mitotic spindle so that one copy/chromosome is distributed to each daughter cell when the cell divides.

centromere

______ contain repeated nucleotide sequences that enable the ends of chromosomes to be replicated and also cap the end of the chromosome to prevent mistaking it for being broken DNA that needs repair.

telomere

Interphase chromosomes occupy their own _______ _______ within the nucleus.

distinct territories

What is the nucleolus?

A non membrane-bound structure composed of proteins and nucleic acids found within the nucleus. It is the most prominent structure in an interphase cell.

What is heterochromatin?

It is tightly compacted chromatin. It is a gene-poor and is located mainly around the periphery, immediately under the nuclear envelope.

What are nucleosomes?

They contain DNA wrapped around a core of eight histone molecules.

What are histones?

histones are highly basic proteins abundant in lysine and arginine residues that are found in eukaryotic cell nuclei

\[T/F\] Chromatin is the complex of both classes of protein (histones and non-histone proteins) with nuclear DNA.

True

What are the aspects of the nucleosome core:

8 histone proteins: H2A, H2B, H3, and H4 (each x2)

The double-stranded DNA, 147 nucleotide pairs long, that winds around the histone octamer

What is the nucleosome?

A nucleosome core particle plus one of its adjacent DNA linkers (\~80 nucleotide pairs)

The 147 nucleotide pairs of DNA is wrapped around the nucleosome core:

a. 2.5 turns

b. 1.7 turns

c. 0.7 turns

d. 3.0 turns

1\.7 turns

What charge is the sugar-phosphate backbone of DNA?

negative

What charge is the histone?

positive

\[T/F\] The sequence of DNA matters when wrapping around a histone octamer.

False. It is based on charge, not nucleotide sequence.

The chromatin in human chromosomes is folded into __________?

looped domains

What is the name of the normal, non-packed chromatin in a chromosome?

a. homochromatin

b. heterochromatin

c. alochromatin

d. euchromatin

euchromatin

What is methylation?

A chemical reaction in the body in which a small molecule called a methyl group gets added to DNA, proteins, or other molecules. This occurs for gene sequences via heterochromatin.

What is acetylation?

A reaction that introduces an acetyl functional group (acetoxy group, CH3CO) into an organic chemical compound. This occurs for gene expression via euchromatin.

What is the position effect?

The activity of a gene depends on its position along a chromosome.

K! 1) The specialized DNA sequences that cap the ends of eukaryotic chromosomes are called:

telomeres

K! 2) What structure in an interphase eukaryotic cell contains ribosomal RNA genes?

the nucleolus

K! 3) The DNA in eukaryotic chromosomes is folded into a compact form by interactions with:

proteins

K! 4) What statement about the nucleolus is **false**?

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a. Nucleosomes are DNA wrapped around 8 histone proteins, plus linker DNA.

b. Nucleosomes can be seen in the electron microscope.

c. Nucleosomes are found only in mitotic chromosomes.

d. Nucleosomes represent a fundamental level of chromatin packing.

Nucleosomes are found only in mitotic chromosomes.

K! 5) How do chromatin-remodeling complexes work?

They use the energy from ATP hydrolysis to alter the chromatin structure.

K! 6) The tails of the core histone proteins can be chemically modified by the covalent addition of which chemical group(s)?

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a. Acetyl

b. Phosphate

c. Methyl

d. All of the above

All of the above

K! 7) Histones are an example of a sequence-specific DNA-binding protein. \[T/F\]

False

K! 8) In interphase chromosomes, regions of the chromosome that contain genes being expressed are generally more compact. \[T/F\]

False