Molecular Basis of Inheritance Flashcards

This set of 170 flashcards covers the complete molecular basis of inheritance as described in the lecture transcript, including DNA structure, history, replication, transcription, translation, gene regulation, the Human Genome Project, and DNA fingerprinting.

What is deoxyribonucleic acid (DNA)?

A long polymer of deoxyribonucleotides that acts as the genetic material in most organisms.

What are the two types of nucleic acids found in living systems?

Deoxyribonucleic acid (DNA) and ribonucleic acid (RNA).

What are the primary functions of RNA in living systems?

RNA mostly functions as a messenger, but also acts as an adapter, structural molecule, and in some cases as a catalytic molecule.

How is the length of DNA usually defined?

As the number of nucleotides or the number of base pairs ($bp$) present in it.

How many nucleotides are present in the bacteriophage $\phi \times 174$?

$5386$ nucleotides.

What is the genome size of Bacteriophage lambda?

$48502$ base pairs ($bp$).

What is the length of DNA in Escherichia coli?

$4.6 \times 10^6\,bp$.

What is the haploid content of human DNA?

$3.3 \times 10^9\,bp$.

What are the three components of a nucleotide?

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

What are the two types of nitrogenous bases?

Purines (Adenine and Guanine) and Pyrimidines (Cytosine, Uracil, and Thymine).

Which nitrogenous bases are classified as Purines?

Adenine and Guanine.

Which nitrogenous bases are classified as Pyrimidines?

Cytosine, Uracil, and Thymine.

Which nitrogenous base is unique to DNA and absent in RNA?

Thymine.

Which nitrogenous base is unique to RNA and absent in DNA?

Uracil, which is found in place of Thymine.

How is a nitrogenous base linked to a pentose sugar?

Through an N-glycosidic linkage to the $1'\,C$ of the pentose sugar.

What structure is formed when a nitrogenous base is linked to a pentose sugar?

A nucleoside.

How is a phosphate group linked to a nucleoside?

Through a phosphoester linkage to the $OH$ of the $5'\,C$ of the pentose sugar.

What is the linkage that joins two nucleotides in a dinucleotide?

$3'-5'$ phosphodiester linkage.

What groups constitute the backbone of a polynucleotide chain?

Sugars and phosphates.

Where do nitrogenous bases project in a polynucleotide chain?

They project from the sugar-phosphate backbone.

What is the structural difference in the ribose sugar of RNA compared to DNA?

In RNA, every nucleotide residue has an additional $-OH$ group present at the $2'$ position.

What is the chemical name for Thymine?

$5\text{-methyl uracil}$.

Who first identified DNA as an acidic substance in the nucleus in 1869?

Friedrich Meischer, who named it 'Nuclein'.

When did Watson and Crick propose the Double Helix model for DNA?

In 1953.

Whose X-ray diffraction data was used by Watson and Crick to model DNA?

Maurice Wilkins and Rosalind Franklin.

What is Chargaff's rule regarding double-stranded DNA?

The ratios between Adenine and Thymine and Guanine and Cytosine are constant and equal one.

What is the meaning of 'complementary' in the context of DNA strands?

If the sequence of bases in one strand is known, the sequence in the other strand can be predicted due to specific base pairing.

What is the polarity relationship between the two chains of DNA?

They have anti-parallel polarity, meaning if one chain is $5' \rightarrow 3'$, the other is $3' \rightarrow 5'$.

How many hydrogen bonds form between Adenine and Thymine?

Two hydrogen bonds.

How many hydrogen bonds form between Guanine and Cytosine?

Three hydrogen bonds.

Why is the distance between the two strands of the DNA helix approximately uniform?

Because a purine always comes opposite to a pyrimidine.

In what fashion are the two chains of DNA coiled?

In a right-handed fashion.

What is the pitch of the DNA helix?

$3.4\,nm$.

How many base pairs are roughly contained in each turn of the DNA helix?

$10\,bp$.

What is the approximate distance between base pairs in the DNA helix?

$0.34\,nm$.

What additional factor, besides hydrogen bonds, confers stability to the DNA helical structure?

The plane of one base pair stacks over the other in the double helix.

What is the Central Dogma in molecular biology proposed by Francis Crick?

Genetic information flows from $DNA \rightarrow RNA \rightarrow Protein$.

In which organisms can the flow of genetic information be reversed (RNA to DNA)?

In some viruses.

What is the total length of DNA in a typical mammalian cell?

Approximately $2.2\,metres$.

What is the typical dimension of a mammalian nucleus?

Approximately $10^{-6}\,m$.

How is DNA held in the 'nucleoid' of prokaryotes like E. coli?

Positively charged proteins hold the negatively charged DNA in large loops.

What are histones?

A set of positively charged, basic proteins found in eukaryotes around which DNA is wrapped.

Which basic amino acid residues are histones rich in?

Lysine and Arginine.

What is a histone octamer?

A unit comprising eight histone molecules around which DNA is wrapped.

What is a nucleosome?

A structure consisting of negatively charged DNA wrapped around a positively charged histone octamer.

How many base pairs of DNA are typically contained in one nucleosome?

$200\,bp$.

What is chromatin?

The repeating unit of nucleosomes in the nucleus, seen as thread-like stained bodies.

How does chromatin appear under an electron microscope?

As a 'beads-on-string' structure.

What are non-histone chromosomal (NHC) proteins?

Additional proteins required for the higher-level packaging of chromatin into chromosomes.

What is euchromatin?

Loosely packed, lightly staining regions of chromatin that are transcriptionally active.

What is heterochromatin?

Densely packed, dark staining regions of chromatin that are transcriptionally inactive.

When did Frederick Griffith conduct his transformation experiments?

In 1928.

Which bacterium was used by Frederick Griffith in his experiments?

Streptococcus pneumoniae (also known as pneumococcus).

In Griffith's experiment, why did the S strain produce smooth colonies?

Because it had a mucous (polysaccharide) coat.

What happened when mice were injected with heat-killed S strain and live R strain bacteria?

The mice died and living S bacteria were recovered from them.

What did Griffith conclude about the transformation of R strain bacteria?

Some 'transforming principle' was transferred from the heat-killed S strain to the R strain, enabling it to become virulent.

Who determined the biochemical nature of the 'transforming principle' between 1933-44?

Oswald Avery, Colin MacLeod, and Maclyn McCarty.

Which enzyme inhibited the transformation process in Avery's experiments?

DNase (DNA-digesting enzyme).

Who provided unequivocal proof that DNA is the genetic material in 1952?

Alfred Hershey and Martha Chase.

What organisms did Hershey and Chase use for their experiment?

Bacteriophages and Escherichia coli bacteria.

Why did Hershey and Chase use radioactive phosphorus ($^{32}P$)?

To label DNA, because DNA contains phosphorus but protein does not.

Why did Hershey and Chase use radioactive sulfur ($^{35}S$)?

To label proteins, because proteins contain sulfur but DNA does not.

What was the result for bacteria infected with phages containing radioactive DNA?

The bacteria were radioactive, indicating DNA passed from the virus to the bacteria.

What was the result for bacteria infected with phages containing radioactive protein?

The bacteria were not radioactive, indicating protein did not enter the bacteria.

Name two viruses where RNA acts as the genetic material.

Tobacco Mosaic Virus and QB bacteriophage.

What are the four criteria for a molecule to act as genetic material?

Must be able to replicate, be stable, provide scope for slow mutations (evolution), and express itself as Mendelian Characters.

Why is DNA chemically more stable than RNA?

DNA lacks the reactive $2'-OH$ group found in RNA and contains Thymine instead of Uracil.

Which molecule is preferred for the storage of genetic information?

DNA, because of its chemical and structural stability.

Which molecule is preferred for the transmission of genetic information?

RNA.

What was likely the first genetic material?

RNA.

What is the Watson-Crick model for DNA replication called?

Semiconservative replication.

What does semiconservative replication mean?

Each daughter DNA molecule has one parental strand and one newly synthesized strand.

Who performed the experiment to prove semiconservative replication in 1958?

Matthew Meselson and Franklin Stahl.

What isotope of nitrogen was used by Meselson and Stahl to label DNA?

$^{15}N$ (a heavy isotope, not radioactive).

How was the DNA of different densities separated in Meselson and Stahl's experiment?

By centrifugation in a cesium chloride ($CsCl$) density gradient.

What was the density of E. coli DNA after one generation (20 minutes) in $^{14}N$ medium?

Hybrid or intermediate density.

What was the DNA composition of E. coli after two generations (40 minutes) in $^{14}N$ medium?

Equal amounts of hybrid DNA and 'light' DNA.

Who used radioactive thymidine to prove semiconservative replication in Vicia faba?

Taylor and colleagues in 1958.

What is the main enzyme involved in DNA replication?

DNA-dependent DNA polymerase.

What is the average rate of polymerization in E. coli?

Approximately $2000\,bp$ per second.

What dual purpose do deoxyribonucleoside triphosphates serve in replication?

They act as substrates and provide energy for the polymerization reaction.

What is a replication fork?

The small opening of the DNA helix where replication occurs.

In what direction does DNA polymerase catalyze polymerization?

Only in the $5' \rightarrow 3'$ direction.

Which strand has continuous replication during DNA synthesis?

The template strand with polarity $3' \rightarrow 5'$.

What are the discontinuously synthesized DNA fragments joined by?

DNA ligase.

What is the origin of replication?

The specific region in DNA where replication initiates.

In eukaryotes, at which phase of the cell cycle does DNA replication occur?

The S-phase.

What results from a failure in cell division after DNA replication?

Polyploidy.

Define transcription.

The process of copying genetic information from one strand of DNA into RNA.

Which nitrogenous base pairs with Adenine during transcription?

Uracil.

What are the three regions of a transcription unit in DNA?

A Promoter, the Structural gene, and a Terminator.

What is the polarity of the template strand in transcription?

$3' \rightarrow 5'$.

What is the coding strand in transcription?

The DNA strand with polarity $5' \rightarrow 3'$ that has the same sequence as the RNA (except it has T instead of U).

Where is the promoter located in a transcription unit?

Towards the $5'\text{-end}$ (upstream) of the coding strand.

Where is the terminator located in a transcription unit?

Towards the $3'\text{-end}$ (downstream) of the coding strand.

What is a cistron?

A segment of DNA coding for a polypeptide.

Contrast monocistronic and polycistronic structural genes.

Monocistronic (mostly eukaryotes) codes for one polypeptide; polycistronic (mostly bacteria) codes for multiple.

What are exons?

Coding sequences or expressed sequences that appear in mature or processed RNA.

What are introns?

Intervening sequences that do not appear in mature or processed RNA.

Name the three major types of RNA in bacteria.

$mRNA$ (messenger), $tRNA$ (transfer), and $rRNA$ (ribosomal).