B DNA and Cell Division Flashcards
What is the structure of DNA?
- A polymer made of nucleotides that contain a pentose sugar (deoxyribose), a phosphate group and a base.
- Mononucleotides are joined by condensation reactions, forming a phosphodiester bond between the pentose sugar of one monomer and the phosphate group of another monomer.
- A DNA molecule consists of two polynucleotide strands, twisted to form a double helix, each helix has a sugar-phosphate backbone, with the central bases joined by hydrogen bonds.
What are the two different types of bases in DNA?
- Thymine and cytosine are pyrimidines.
- Guanine and adenine are purines.
- Adedine and thymine form 2 hydrogen bonds between each other, cytosine and guanine form 3 hydrogen bonds between each other.
How does the structure of DNA correspond to its function?
- Bases in sequence code for amino acids, therefore the base sequence stores genetic information.
- DNA is very large so can store a massive amount of information.
- DNA’s helix makes it compact.
How is DNA extracted from cells?
DNA purification:
1. Grind the sample to break down cell walls.
2. Add to a test tube and mix with a detergent to break down the nuclear envelopes.
3. Add protease (to break down proteins that are attached to DNA).
4. Add ethanol (causes DNA to precipitate as a white solid).
5. Extract DNA with a glass rod.
Compare eukaryotic and prokaryotic DNA:
- Eukaryotic DNA contains exons and introns, whereas prokaryotic DNA only contains exons.
- Eukaryotic DNA and histones form chromosomes, whereas prokaryotic DNA cannot form chromosomes.
- In eukaryotes DNA is also found in mitochondria and chloroplasts (as short, circular DNA that is not associated with chromosomes).
- Some prokaryotes have small, circular DNA that lies free in the cytoplasm called plasmids, which replicate independently.
How are chromosomes formed?
- DNA condenses into chromatin, by wrapping tightly around histones, enabling it to fit into the nucleus.
- Chromatin folds together to form a chromatid.
- Sister chromatids are then joined at the centromere to form a chromosome.
- The end of each chromatid has a telomere which stops the DNA from degrading.
What is binary fission?
The process by which prokaryotic cells divide:
1. The single, circular DNA is replicated (any plasmids present are also replicated).
2. The cytoplasm divides, forming two daughter cells, these may have different numbers of plasmids, but each have a copy of the circular DNA.
How do viruses replicate?
- They are not cells so do not undergo cell division.
- They inject their nucleic acid into host cells and the host cell then replicates the virus particles.
What is interphase?
- The longest stage of the cell cycle that gets the cell ready to divide.
- During G1 (growth 1) the cell grows in size and synthesises new organelles and proteins.
- During synthesis, DNA is replicated.
- During G2 the cell continues to grow in size and new proteins/organelles are synthesised.
What are the checkpoints during interphase?
- G1 checkpoint checks cell size, that there are enough proteins and organelles and that DNA isn’t damaged.
- G2 checkpoint checks that DNA has been replicated correctly.
What occurs during mitosis?
The nucleus divides in two - DNA has already been replicated is the mass of DNA and the mass of the cell remains unchanged.
What occurs during cytokinesis?
The cell divides in two, creating two genetically identical daughter cells.
Which cells enter the G0 phase?
- Cells that don’t divide eg. differentiated cells
- Cells that fail to pass the G1 checkpoint (which prevents mutated cells from dividing).
What are the functions of mitosis?
- Growth (from a zygote to a multicellular organism)
- Repair (damaged cells can be quickly replaced by genetically identical cells)
- Reproduction (enables asexual reproduction and produces gametes from haploid cells for sexual reproduction).
What is the mitotic index?
The ratio between the number of cells undergoing mitosis in the population, with the total population of cells.
