DNA Organization in Eukaryotes and Prokaryotes

These flashcards cover key concepts related to DNA organization, structure, and replication in both eukaryotic and prokaryotic cells, as outlined in the lesson notes.

What must DNA do to fit inside a 10μm nucleus?

It must be compacted through multiple levels of coiling and structuring

What are the three steps in compacting DNA?

• Step 1: DNA wraps around histone proteins to form nucleosomes

• Step 2: Nucleosomes coil together to form solenoids

• Step 3: During cell division, fibers further supercoil into chromosomes

What are histones?

Positively charged proteins that attract negatively charged DNA

How is a nucleosome formed?

DNA wraps around 8 histones, containing about 147 base pairs

What two forms can chromatin exist in?

• Euchromatin (loosely packed, active for transcription)

• Heterochromatin (tightly packed, inactive storage)

Why is chromatin dynamic?

Cells must turn genes on/off depending on their function

What form does DNA take during interphase?

DNA remains as chromatin to allow gene expression

• Reading DNA to make RNA and proteins

What happens to chromatin during mitosis?

Chromatin condenses into X-shaped chromosomes for efficient cell division

Why do chromosomes only appear during mitosis and not interphase?

• Because chromosomes are tightly packed forms of DNA, making it easier to divide DNA evenly during mitosis

• In interphase, DNA stays loose as chromatin to allow access for transcription and replication

DNA organization in prokaryotes

• DNA is circular

  • Linear chromosomes exist but are rare

• No nuclear membrane/nucleus, so DNA is less tightly bound and more accessible because it is in the cytoplasm

• Lacks histones

What are plasmids?

Smaller, circular fragments of DNA that float throughout the cell in prokaryote cells that self-replicate

• They carry extra genes

What is conjugation?

When one bacterium transfers a plasmid to another, and the recipient bacterium can incorporate the plasmid for genetic variation

What is supercoiling in prokaryotes and why is it necessary?

The tightening and twisting of circular DNA into a tight ball to fit inside the cell by enzymes called topoisomerases

• Necessary because bacterial cells lack histones so they must compact DNA differently

What is the main problem in the DNA replication of eukaryotes?

DNA polymerase cannot fully replicate the ends of chromosomes

• Over time, this can cause the loss of essential genes

Why can’t the ends of chromosomes be fully replicated?

Because of a lagging strand replication issue—the DNA polymerase leaves a small section at the end unreplicated

What is the lagging strand, and how is it replicated?

The lagging strand is built in short sections called Okazaki fragments, made in the 5’ to 3’ direction away from the replication fork

Why can't the final RNA primer be replaced with DNA on the lagging strand?

There’s no free 3’ OH group for DNA polymerase to attach new DNA to, so the last RNA primer is removed but not replaced

What is the role of telomeres?

Telomeres prevent the loss of essential genes during DNA replication

• They consist of the sequence 5’ TTAGGG 3’

  • They are non-coding DNA and do not contain G’s

What happens to telomeres with each replication cycle?

They get shorter, which is associated with cell aging and eventual cell senescence

What is the significance of plasmids in bacteria?

They carry extra genes, often for antibiotic resistance and can be transferred between bacteria.

What is the main issue during eukaryotic DNA replication?

DNA polymerase cannot fully replicate the ends of chromosomes, leading to potential gene loss.

What is the function of telomerase and where is it found?

• Telomerase is an enzyme that adds new telomere DNA, keeping telomeres long

• Found in germ cells (sperm/egg), stem cells, and some white blood cells

Place the following structures in order of size, from smallest to largest: solenoid, nucleosome, chromosome, histone

Histone (small protein), nucleosome (DNA wrapped around histones), solenoid (a group of 6 nucleosomes), chromosome (tightly packed structure of DNA)

Why can DNA wrap tightly around histones?

• DNA is negatively charged (from phosphate groups)

• Histones are positively charged (from amino acids)

• Opposite charges attract, letting DNA wrap tightly

• Helps compact DNA into the nucleus

Differentiate DNA packing in bacteria and eukaryotes

• Chromosome

• Packing (tight or loose)

• Histones

• Where it is found

• How it folds

What are the five functions of teleomeres?

• Protect the DNA from losing valuable coding (genes) info during replication

• Prevent chromosome ends from fusing to other chromosomes

• Prevent DNA degradation from nucleases (enzymes)

• Know the difference between DNA breaks vs the ends of chromosomes

• Determine how many times a cell can divide and therefore determine lifespan of an organism

What happens when telomeres become too short?

• The cell cannot divide anymore

• Enters a state called senescence (cell aging)

• The cell stays alive but cannot grow, divide, or repair

Hayflick limit

• The number of times a normal cell can divide

  • Usually cells divide around 50 for humans

• Different for every species

Explain cancer cells and how they divide forever

• They make a lot of telomerase, so their telomeres do not get shorter

  • This allows them to divide forever out of control

• This gives them immorality unlike normal somatic cells who do not have telomerase which allows for senescence and cell death

How cancer therapy works

They try to block telomerase so cells can die off naturally

Why is telomerase needed in germ cells?

• Germ cells are sperm and egg cells

• They pass on DNA to the next generation

• Without telomerase, their telomeres would shorten too much, risking gene loss

• Telomerase keeps telomeres long for healthy offspring

Pros and cons of telomerase therapy

Pros:

• Can slow aging

• Improve cell health and tissue repair

Cons:

• Too much telomerase can cause a risk of uncontrolled cell growth and division

  • This can lead to cancer

• Cells may not die which is dangerous

Cancer therapy: Shortening telomeres

• Cancer cells often have active telomerase = unlimited division

• Cutting telomere length can limit their lifespan

• Eventually, cancer cells will stop dividing and die

• Could slow or stop tumor growth

Problem with cloning from older DNA (ex. Dolly the Sheep)

• Older cells = shorter telomeres

• Cloned organisms may start life with “aged” DNA

  • Can cause early aging, health problems, and shorter lifespan (Dolly the Sheep)