Session 1 - DNA Structure

Question 1

What are nucleic acids?

Answer 1

polymers (large molecule, composed of many repeated subunits) of nucleotides. The 2 main nucleic acids are DNA and RNA.

Question 2

What is a nucleotide composed of?

Answer 2

a phosphate group, a pentose (5 carbon) sugar, and a nitrogenous base (guanine, cytosine,
adenine, thymine or uracil)

Question 3

What are the two basic structures of nitrogenous bases?

Answer 3

Purines and pyrimidines. Purines are heterocyclic aromatic organic compounds containing 4 nitrogen atoms and two carbon rings. Purines are are adenine and guanine. Pyrimidines are heterocyclic aromatic organic
compounds containing 2 nitrogen atoms and only one
carbon ring. Pyrimidines are cytosine, thymine, and uracil.

Question 4

How are nucleotides formed?

Answer 4

Through condensation reactions (reactions that give off water). A nucleotide is formed when a phosphoric acid and a base are chemically bonded to a sugar molecule (this bond is covalent). Water is given off when both the phosphate group and base group are joined.

Question 5

How are polynucleotides formed?

Answer 5

When nucleotides are linked via condensation reactions. The 5’-phosphate group of one nucleotide attaches to the sugar of another nucleotide (at the 3’-hydroxyl group) (this is a covalent bond).

Question 6

How is double helix of DNA formed?

Answer 6

Purines and pyrimidines are attracted to each other by hydrogen bonds and this bond causes two nucleic acid chains to join together. But in order for the chains to join they must run in anti-parallel directions (one of them is upside down). The sugars and phosphates of these nucleic acids chains form “very rigid backbones” because they are bonded by covalent bonds.

Question 7

How are DNA strands complementary?

Answer 7

One strand (the sense strand) has a certain sequence of
bases, while the other strands (antisense strand) has the complementary sequence. This is because of the base pair rule: A can only bond to T and C can only bond to G. And 2 bonds form between A-T but 3 bonds form between C-G.

Question 8

Describe the helix of DNA.

Answer 8

The helix forms because the phosphate/ribose backbone of DNA is hydrophilic (water loving), so it orients itself outward toward the solvent, while the relatively hydrophobic bases bury themselves inside. The helix twists so as to form the most stable energy configuration.

Question 9

How does RNA differ from DNA?

Answer 9

The sugar is ribose (instead of deoxyribose in DNA), and there is no thymine - it is replaced by the base uracil. RNA also forms a single strand - it is much shorter than DNA - but it can form loops via complementary base pairing.

Question 10

What are the three types of RNA?

Answer 10

Messenger RNA, Ribosomal RNA, and Transfer RNA. mRNA is a transcript copy of a gene (codes for protein), rRNA is the catalytic component of ribosomes, tRNA transports amino acids to the ribosome.

Question 11

What is the structure of chromosomes and how many do humans have?

Answer 11

Chromosomes are located inside the nucleus, they have a central centromere and two sister chromatids. Chromosomes make up DNA and humans have 46 chromosomes (23 pairs) in each cell.

Question 12

What is a pentose sugar?

Answer 12

A sugar with 5 carbons. It is structured like a Pentagon: the top point is the O group: the next four points, when you move clockwise, are carbon 1, carbon 2, carbon 3, carbon 4. Carbon 5 is located between carbon 4 and the bond with the phosphate. When nucleotides bond, the phosphate bonds with the 3rd carbon (3 is OH).

Question 13

What are the sense and antisense strands?

Answer 13

When replication occurs, the sense strand is the original, and the antisense strand is the new strand.

Question 14

What is chromatin?

Answer 14

Chromatin is a combination of DNA, histone proteins, and other proteins that makes up chromosomes. It is found inside the nuclear envelope of eukaryotic cells.

Question 15

What does chromatin do?

Answer 15

packages DNA into a smaller volume to fit in the cell
and nucleus, strengthens the DNA to allow mitosis and meiosis to occur, and controls gene expression and DNA replication.

Question 16

What are the two distinguishable areas of chromosomes?

Answer 16

Heterochromatin and euchromatin. Heterochromatin (condensed) is tightly packed and often indicates the DNA region is transcriptionally inactive. Euchromatin (extended) is high in gene concentration and often indicates higher amounts of transcription is occurring.

Question 17

How does chromatin become chromosome?

Answer 17

The DNA wraps around a bundle of 8 histone proteins (2 x 4 types) to form a nucleosome. An additional histone (H1) helps the chromatin fibre form looped domains. The looped domains are attached to a scaffold non-histone protein. The looped domains themselves fold repeatedly. Repeated folding produces the condensed chromosome.

Question 18

Describe how histone tails can change charge and how this impacts transcription.

Answer 18

Typically the histone tails have a positive charge and
hence are attracted to negatively charged DNA. Adding an acetyl group to the tail (acetylation) neutralises the charge, making DNA less tightly coiled and increasing transcription (euchromatin). Adding a methyl group to the tail (methylation) maintains the positive charge, making DNA more coiled and reducing transcription (heterochromatin)

Question 19

What is DNA methylation?

Answer 19

DNA methylation is an epigenetic mechanism
involving the transfer of a methyl group onto the C5
position cytosine bases. During development, differentiated cells develop a stable and unique DNA methylation pattern that regulates tissue-specific gene expression.

Question 20

What is epigenetics?

Answer 20

Epigenetics is the study of how chemical and environmental triggers influence the gene expression patterns. Epigenetic analysis shows that DNA methylation patterns may change over the course of a lifetime. It is initially influenced by heritability but is not genetically pre-determined (identical twins may have different DNA methylation patterns over their lifetime but they begin relatively the same). Different cell types in the same organism may have markedly different DNA methylation patterns. Environmental factors (e.g. diet, pathogen exposure, etc.) may influence the level of DNA methylation within cells. Direct methylation of DNA (as opposed to the histone tails) can also affect gene expression patterns.

Question 21

What is transcription and why does it occur?

Answer 21

Transcription is when a gene’s DNA sequence is copied to make an RNA molecule. This is the first step in gene expression, in which information from a gene is used to construct a functional product such as a protein.