Nucleic Acids - The Human Genome

Question 1

What is the genome?

Answer 1

All the genetic information of an organism

Question 2

How many nucleotides in humans? How many protein coding genes does this make up?

Answer 2

3.2 billion nucleotides making up 20,000 protein coding genes

Question 3

What is a genome divided into?

Answer 3

• 2 sex chromosomes = XX and XY
• 44 autosomes = non -sex chromosomes
• (37 genes on circular mitochondrial chromosomes)

Question 4

Why do mitochondria have their own separate set of genes?

Answer 4

Evolved from bacteria (endosymbiosis) which had roughly the same size, shape and had circular DNA

Question 5

Why do mitochondria have their own separate set of genes?

Answer 5

Evolved from bacteria (endosymbiosis) which had roughly the same size, shape and had circular DNA

Question 6

What is a karyotype and how is ordered?

Answer 6

The number and visual appearance of chromosomes.
Autosomes numbered in order of size (1 is biggest).

Question 7

What percentage of DNA is coding DNA?

Answer 7

1%

Question 8

What does non-coding DNA include?

Answer 8

• Genes for other RNAs
• Promoter regions
• Introns
• Tandem repeats
• Telomeres
• Pseudogenes
• Viral DNA
• “Junk DNA”

Question 9

What are tandem repeats?

Answer 9

• Repeated short DNA sequences which occur due to polymerase slippage.

Question 10

What can tandem repeats be used in? Why?

Answer 10

• Used in forensic DNA testing and paternity testing
• Number of repeats are variable and differ between individuals
• Easier way of testing than using the whole genome.

Question 11

What are telomeres?

Answer 11

Long stretches of DNA at the end of chromosomes

Question 12

How are telomeres the basis of aging?

Answer 12

Telomeres shorten after each replication, at senescence, telomeres are so short that the specific cell cannot divide anymore and any damage incurred cannot be repaired.

Question 13

What is special about cancer cell telomeres?

Answer 13

Cancer cells are good at keeping their telomeres long

Question 14

What are pseudogenes

Answer 14

A DNA sequence that resembles a gene but has been mutated into an inactive form over the course of evolution

Question 15

What is viral DNA?

Answer 15

We have more viral DNA than we do our own coding DNA. Most from retroviruses that replicate by inserting their genome into our DNA

Question 16

What is “junk DNA”

Answer 16

Has no obvious purpose yet

Question 17

What are haploid cells?

Answer 17

n=23 one copy of each chromosome
e.g. sperm and eggs

Question 18

What are polyploid cells? Examples?

Answer 18

cells that copy their DNA several times (2n, 4n, 8n) without division
- Megakaryocytes (parent cell of platelets)
- cardiomyocytes (unknown why as it impairs heart regeneration after damage.

Question 19

What are diploid cells?

Answer 19

2n= 46 two copies of each chromosome (half maternal and half paternal)
most of the cells in the body

Question 20

How does inactivation of the X chromosomes in women take place?

Answer 20

• Takes place in the early embryo where inactivation is by random selection in each cell
• Each cell generates a clone with the same X active

Question 21

What is a Barr body?

Answer 21

Inactivated X chromosome which shrinks and is pushed to the edge of the nucleus

Question 22

When and why do red blood cells lose their nucleus?

Answer 22

• Lose their nucleus during maturation
• Makes room for haemoglobin
• Maintains a concave shape to maximise oxygen diffusion
• provides red blood cells with the flexibility to squeeze through capillaries

Question 23

What is an example of a species with a more compact genome?

Answer 23

Pufferfish have 10% more coding DNA, 90% non coding DNA

Question 24

What is an example of species which have a different ability to repair cardiac myocytes?

Answer 24

• Zebra fish have 2n chromosomes per cardiomyocyte throughout life and can repair heart damage as adults
• Newborn mice have 2n chromosomes per cardiomyocyte and can repair heart damage while newborn

Question 25

What is an example of a species that have different shaped chromosomes?

Answer 25

Bacteria have ring chromosomes (we have linear chromosomes)

Question 26

What is an example of a species with a different number of chromosomes? (An example of synteny)

Answer 26

Chimps have 2n=48 chromosomes (our chromosome 2 is a fusion of chimp chromosomes 12 and 13)

Question 27

what is synteny?

Answer 27

- When long DNA sequences are present in the same order in two different species - Occur as a result of chromosome breakage and re-joining (translocation) throughout evolution

Question 28

What percentage identical are the genomes of any two individuals? How many base pairs difference is this?

Answer 28

- 99.6% identical - leaves about 24 million base pairs of difference

Question 29

What is the genotype?

Answer 29

unique DNA sequence responsible for a trait

Question 30

What is the phenotype?

Answer 30

observable expression of the trait e.g. tall, curly hair

Question 31

What is a single nucleotide polymorphism (SNP)?

Answer 31

a variation at a single position in a DNA sequence among individuals; if more than 1% of the population does not carry the same nucleotide at a specific position in the DNA sequence = SNP

Question 32

What is a single nucleotide polymorphism (SNP)?

Answer 32

a variation at a single position in a DNA sequence among individuals; if more than 1% of the population does not carry the same nucleotide at a specific position in the DNA sequence = SNP

Question 33

How can a gene be described if it has an SNP in the coding region?

Answer 33

The gene is described as having more than one allele, some may alter our relative disease risks *SNPs can also occur in non-coding DNA

Question 34

What two factors can result in multifactorial diseases? Example?

Answer 34

- SNPs in multiple genes (polygenic) - The environment e.g. Hypertension

Question 35

What is aneuploidy? How does it occur? Example?

Answer 35

wrong number of chromosomes often due to non-dysjunction (failure of chromosomes to properly separate) during meiosis e.g. Downs Syndrome (Trisomy 21)

Question 36

What is an example of a disease that can be caused by translocations in somatic cells?

Answer 36

Chronic myeloid leukaemia (due to formation of Philadelphia chromosome t(9;22))

Question 37

What is an example of a disease that can be caused by translocations in germ cells?

Answer 37

Rarer forms of Downs Syndrome (less severe - where a big chunk of chromosome 21 can get stuck onto another chromosome so you almost have 3 chromosome 21s)

Question 38

What is an example of a disease caused by single gene mutations? How does infection occur?

Answer 38

Cystic fibrosis - mutation in CFTR gene which is a chloride ion transporter so build up of thick mucus which cannot be cleared = infections

Question 39

What are single gene mutations?

Answer 39

- Result from a single mutation in a single gene - little influence from other genes or environment on phenotype

Question 40

Where do tandem repeats normally occur?

Answer 40

In non-coding regions of DNA, but can occur in coding regions which alters protein structure

Question 41

What is an example of a disease due to tandem repeats? How can it become more severe with inheritance?

Answer 41

- Huntington's Disease = increased CAG repeats in the Huntington protein - With inheritance, the genes for the disease get longer so disease becomes more severe in subsequent generations

Question 42

How do diseases from mitochondrial DNA occur?

Answer 42

- Mutations inherited in mitochondrial DNA - rare and severe - 37 mitochondrial genes, if there is a mutation in these, it is normally incompatible with life

Question 43

Why can mitochondrial DNA only be inherited from the mother?

Answer 43

only the egg contains mitochondria, the sperm mitochondria is in the tail, the head which releases genetic material fuses with the egg and leaves the tail behind