W3L1 - Origin of eukaryotes Pt 2/2

Question 1

Prokaryotes and Humans: Chromosomes and Length

Answer 1

Prokaryotes: 1 circular about 1mm long

Eukaryotes: 46 linear about 3000mm long

Question 2

What is the “end problem”. (1)

What is the solution to the “end problem” (2)

Answer 2

“End Problem”:

• Linear DNA would get shorter after each replication

Solution:

• Telomeres: “End Structure”
• Telomeres add nonsense DNA to end of the chromosome to compensate for it being shorter each time. By tying sacrificial DNA onto the ends, the RNA primer can attach to and then later be deleted without losing crucial genes in the middle.

Question 3

Two properties of DNA discussed in the lecture

Answer 3

1. DNA ends are vulnerable
   - Linear chromosomes have caps (telomeres) on DNA end that protect them from damage (fraying)
2. DNA continuously loses its ends because DNA synthesis must start with a little piece of RNA (“RNA Primer”)
   - Linear chromosomes are continuously extended by adding ‘sacrificial’ DNA to the ends

Question 4

Experiment that showed that telomeres are essential to have linear chromosomes

Answer 4

• Deleted genes for telomere formation in Fission Yeast (3 linear chromosome eukaryote)
• Yeast nearly all died and had few survivors
• Importantly, chromosomes had become circular (Grow ok but unable to have sex)

>>> Can only have linear chromosomes when telomeres are present

Question 5

The link between telomeres and ageing (2)

Answer 5

• Telomeres get shorter as cells age
• Telomeres get too short and we lose “Program”

Question 6

Is Telomere rejuvenation the fountain of youth?

Answer 6

Probably not. Other factors

Question 7

Why do cloned animals have short telomeres?

Answer 7

Cloned animals have short telomeres like the mature cells they are cloned from

Question 8

What is the link between telomeres and cancer?

Answer 8

• Cancer cells have regained long telomeres
• Allows them to grow and divide rapidly (too rapidly)

Question 9

How many origins do bacterial circular chromosomes have? (1)

What are the implications? (2)

Answer 9

Bacterial circular chromosome has a single replication start point (origin)

• More than one origin would cause chaos by multiple catenation (Russian wedding ring) (Catentation - Knotting)
  
  • Even if it goes faster, the ring would be intertwined
• Single origin limits the time to copy the DNA
  
  • Can do about 3,000 genes in 15 minutes
  • Limits the generation time
  • More genes would require more time to replicate
  • Outcompeted by faster replicators

Question 10

Why do telomeres speed up replication time

Answer 10

By having multiple start points, it allows faster replication even if the chromosome is longer.

Question 11

Why having more genes is better

Answer 11

More complexity of message

Question 12

TLDR:

Properties of circular chromosomes (Origins, Time)

Answer 12

• One origin
• Duplication time for 10⁶ bp is at least 15
  minutes
• Generation time > 15 minutes

Question 13

TLDR:

Properties of linear chromosome (Time, Genome)

Answer 13

• Multile origins reduces duplication time
• Eukaryotic genome can get (much) larger
  
  • More genes > more complexity

Question 14

What are DNA parasites? (3)

Answer 14

• Introns
• Junk DNA that interrupts DNA words in a gene: Must be removed for the sense of a gene to be maintained
• Humans have 207,344 introns

Question 15

Introns must be excised before translations can take place.

Where (1) and how is this done? (3)

Answer 15

• Intron excisions happens in the nucleus

Nuclear envelope

• Partitions the process of intron removal (precedes translation)
• Barrier to prevent precocious translation that would make a dodgy protein if introns are still present
• Nuclear envelope probably evolved in response to introns, which were only permitted by DNA expansion with linear chromosomes.