DNA Packaging

Question 1

DNA is stored within cells what does DNA hold and why must it be packaged?

Answer 1

• DNA is stored within cells
• DNA holds all the essential genetic information for cell function
  o This must be replicated and passed on to new cells during cell division
  o DNA must be able to be “read” to produce functional proteins
• DNA molecules can be very long- the DNA in a single human cell would stretch to a length of about 2 metres
• Therefore, DNA must be packaged in an ordered and organised way so that it can carry out the required function within a cell and to fit

Question 2

How are eukaryotes defined?

Answer 2

By having a distinct nucleus

Question 3

Tell me about the DNA within a eukaryotic cell?

Answer 3

o Contained within the nucleus
o Bound to histone proteins to enable packaging into linear chromosomes
o Contains large amounts of non-coding and repetitive DNA

Question 4

What type of organelles do eukaryotes contain? Give an example and what it codes to produce?

Answer 4

Typically contain membrane-bound organelles:
o Including mitochondria- which has its own genome (this mostly codes for the protein involved in ATP production)
o Mitochondria is responsible for making ATP the energy source for the cell

Question 5

Draw ATP and tell me about it

Answer 5

Question 6

Is mitochondrial DNA linear or circular?

Answer 6

Circular (resembles bacterial DNA)

Question 7

How is DNA packaged into eukaryotic cells?

Answer 7

• DNA double helix roughly 2nm wide (up to 2m long)
• Proteins called Histones bind and DNA wraps around it to form nucleosomes
• Nucleosomes coiled into fibred names chromatin
• Chromatin forms loops
• The condenses and compacts into a chromosome structure

Question 8

Tell me about the human genome and the number of different chromosomes

Answer 8

* 23 pairs of chromosomes
o 22 autosomal
o 1 sex (X or Y)
* Diploid cell- 46 chromosomes (23 pairs)- most humans cells are this
* Haploid cell- 23 chromosomes- sex cells or gametes (egg and sperm)
* Karyotype- image of chromosomes arranged in homologous pairs by size

Question 9

Give examples of prokaryotes?

Tell me about DNA within prokaryotic cells?

Answer 9

**Prokaryotic cells e.g., bacteria and archaea **

The DNA within a prokaryotic cell is:
o Found freely in the cytoplasm within the nucleoid
o Generally, have one chromosome found in a circular in shape
o Most often not bound with proteins, does not form chromatin (exception of some archaea)
o Contains little repetitive DNA and no introns (prokaryotes have a compact genome)
o DNA still needs to be coiled to fit into cell

Question 10

Prokaryotic cells can contain extrachromosomal plasmids, what does this mean?

Answer 10

o Capable of self-replication
o Allow exchange of genetic material with other bacteria
o Give cell virulence and antibiotic resistance

Question 11

What is supercoiling?

Answer 11

This is where it is twisted beyond double helix to fit into prokaryotic cells

Example

Question 12

Summary

Answer 12

• Without packaging, DNA molecules would be far too long to fit into cells
• Must be ordered so that it can still carry out its function within a cell
• Eukaryotic cells
  o DNA binds to histones > nucleosomes> chromatin fibres> condensed into linear chromosomes
  o Stored in nucleus
  o Humans- 23 pairs of homologous chromosomes
• Prokaryotic cells:
  o Single chromosomes is circular
  o DNA is supercoiled and found in an area of the cytoplasm called the nucleoid