MCG - Molecular Basis of Inheritance

Question 1

How was it confirmed that DNA was the genetic information of the cell?

Answer 1

A bacteriophage (a virus that infects bacteria) had its DNA and protein radioactively labelled.

The phage was allowed to infect a bacterium, and then the radioactivity measurements were checked.

It was found that the label for DNA was abundant in the progeny phage, thus, it was DNA that was passed on.

Question 2

What three groups are amino acids usually composed of?

Answer 2

• sugar: deoxyribose or oxyribose
• phosphate (PO4 3-)
• nitrogenous bases

Question 3

Name the purines.

Answer 3

Guanine and Adenine.

Remember, GUardian Angels are PURE.

Question 4

Name the pyrimidines.

Answer 4

Cytosine, Thymine and Uracil.

Remember,
the PYRAMIDs are a sight (Cyt) To See.

Question 5

What is the difference between a ribose and a deoxyribose sugar?

Answer 5

ribose = has an OH at 2’ position on sugar ring

deoxyribose = has an H (not OH) at 2’ position on sugar ring

Question 6

Nucelotides can come in three phosphorylated forms.

Which form is used for DNA formation?

Answer 6

The tri-phosphate version (ATP, GTP, CTP, TTP).

Question 7

Describe the reaction catalysed by DNA polymerase.

Answer 7

Two nucleotides are attached to each other by a covalent bond between the hydroxyl group of the first and the phosphate attached to the 5’ carbon of the next nucleotide in line.

This creates a phosphodiester bond.

This bond liberates inorganic phosphate (PPi).

Question 8

What are the three criteria of DNA that need to be met for it to fulfill its purpose as genetic material?

Answer 8

① Must incorporate a feature that determines its role as a store of information.

② Must suggest a replication mechanism, that permits its passage across generations of progeny.

③ Must be capable of undergoing mutation, such that the information carried is altered in a heritable manner.

Question 9

Describe the DNA structure.

Answer 9

It is two polynucleotide chains coiled around a central axis, forming a right hand double helix. The bases are perpendicular to the helical axis.

It spans 3.4 nm for one full twist (which is 10 nucleotides per turn), and has a 2 nm diameter helix.

The strands are antiparallel (5’ to 3’ and 3’ to 5’).

The genetic information is stored as a specific sequence of bases.

Question 10

How is information transferred in DNA?

Answer 10

It is transferred through the semi-conservative method.

The parent nucleotide unwinds.
Two new daughter strands assemble based on base-pairing rules (these rules allow copying and transmission).

This creates a whole new DNA double helix, with one parent strand and one new daughter strand.

Question 11

What is the human genome composed of?

Answer 11

We have 23 pairs of chromosomes = ca. 3.2 x 109 nucleotides
(haploid size)

The average size of protein-coding DNA in a gene is 2,000 bases.

There is coding capacity for > 1 million genes. However, analysis of the human genome sequence reveals only ca. 20,000 genes.

Therefore only ca. 1.1% of the genome encodes protein.

Question 12

What is a key difference between prokaryotic and eukaryotic genes?

Answer 12

In contrast to prokaryotic genes, eukaryotic genes are split by intervening regions (introns).

Coding regions are called exons (1.1% of human genome).

Non-coding regions are called introns (23% of human genome).

Question 13

What three processes are required to convert the pre mRNA into mature RNA that can leave the nucleus?

Answer 13

1. Capping:
   A 7-methylguanosine cap is added to the 5’ end to maintain stability.
2. Polyadenylation:
   A poly-A tail is added to the 3’ end to protect from degradation.
3. Splicing:
   The introns are removed to leave only coding exons.

Question 14

Transposon based repeats make up 45% of the human genome.

Describe transposon based repeats.

Answer 14

They are repeats of DNA that move randomly throughout via copy and past mechanism using RNA intermediate.

They are made up of:

1. LINES (Long Interspersed Elements): some cause disease (such as haemophilia A)
2. SINES (Short Interspersed Elements)

Question 15

Heterochromatin makes up 6% of the human genome.

Describe heterochromatin.

Answer 15

They are repeated sequences that are inactive in transcription (condensed).

It is referred to as satellite DNA, and it makes up our genetic ‘fingerprint’. This can be used in paternity testing and forensic science.

Question 16

We know that the genome specifies non-coding RNA required for splicing and decoding mRNA.

What are some RNA that help with mRNA processing?

Answer 16

Small nuclear RNAs (snRNAs) form complexes with proteins to form small ribonucleoprotein particles (snRNPs), required for splicing pre-mRNAs.

Question 17

We know that the genome specifies non-coding RNA required for splicing and decoding mRNA.

What are some examples of non-coding RNA require for decoding mRNA?

Answer 17

Ribosomal RNA (rRNA) – used to make ribosomes

Transfer RNA (tRNA) – used in translation to form proteins

Question 18

What are long ncRNAs?

Answer 18

The genome also specifies for long ncRNAs (non-coding). We don’t know what most of them do, but some of them are involved in mammalian X inactivation.

Question 19

Explain how long ncRNAs are involved in mammalian X inactivation.

Answer 19

Females have 2 X chromosomes, and only 1 is transcriptionally active. This inactivation occurs through Xic (X-inactivation complex) locus, which contains the Xist (X-inactive specific transcript) gene. It is transcribed to a ncRNA, and then heterochromatin formation occurs.

This occurs in early embryogenesis, so that all descendants have the same inactivation.

Question 20

List some examples of X-linked recessive disorders.

Answer 20

Anhidrotic ectodermal dysplasia (defective sweat glands):
Heterozygous females have random patterns of tissue with and without sweat glands (it affects males a lot more, and no patching).

Red Green colour blindness:
Males are fully colour blind if they carry the mutant allele.
Heterozygous females have mosaic retinas with patches of defective colour perception.

Question 21

What are miRNAs?

Answer 21

They are aother form of ncRNAs which regulate gene expression.

Question 22

Describe the synthesis of miRNA.

Answer 22

1. It is transcribed from the miRNA gene, which gives us a hairpin stem-loop precurosr miRNA
2. It is exported to the cytoplasm and processed by a nuclease.
3. This leaves a single strand of RNA that base pairs with a coding mRNA.

Question 23

Describe the function of miRNA.

Answer 23

The attachment of the miRNA to an mRNA results in:

1. translation repression
2. deadenylation (resulting in mRNA instability)

Question 24

List mutations in miRNA have been implicated in disease.

Answer 24

A mutation in miRNA, miR-96, causes hereditary progressive hearing loss.

Many miRNAs are upregulated or down regulated in cancer, and contribute to its progression.

Question 25

Describe the mitochondrial genome.

Answer 25

Mitochondria contain their own genome (through endosymbiosis), coding for 13 polypeptides, as well as rRNA and tRNA.

It is highly compact, with no introns or repetitive DNA.

Mitochondria are inherited exclusively from the mother – sperm cell mitochondria are excluded from zygote.

It is passed along through matrilineal inheritance – it doesn’t follow any autosomal or sex-linked inheritance patterns.

Question 26

What are key features of mitochondrial cytopathies?

Answer 26

• they can only be passed on by the affected mother, offspring of affected father are normal
• the affected mother would transmit the disorder to all offspring, so can affect both sexes

Question 27

List some diseases associated with mtDNA mutations.

Answer 27

• MELAS – myopathy, encephalopathy, lactic acidosis, stroke-like episodes
• LHON – Leber’s hereditary optic neuropathy