Question

During interphase of the cell cycle, genomic DNA is:

Dispersed (sphagetti-like)

Disintergrated

circular in form

Highly coiled to a thick structure with fat chromatids that look like twinkies

Answer 1

1)

Cell division includes a very important process called mitosis where the nucleus creates a copy of all of its DNA so that each new cell is an exact copy of the parent cell and contains the exact same number of chromosomes. The cell cycle has five phases, but mitosis (nuclear) division occurs in four steps: Prophase, Metaphase, Anaphase, and Telophase. A phase called interphase is not actually part of mitosis, but is the resting phase that the cell is in when it is not dividing.

1. Interphase. A cell spends most of its time in this “in between” phase, performing cell activities like cellular respiration, osmosis, and for plant cells, photosynthesis. During this phase, the DNA is uncoiled and called chromatin. A pair of centrioles are present (but inactive in the cytoplasm) and the nucleolus is visible. At this time the cell grows, the DNA replicates and organelles grow in preparation for cell division. Color the centrioles red and the nuclear membrane yellow. Shade the chromatin blue.

2. Prophase.  This is the first step of mitosis. The nuclear membrane breaks apart and the chromatin condenses into chromosomes. The centrioles form a star shaped structure called the aster and a spindle forms between them. Color the aster pink and the spindle green. In all of the rest of the cell phases, these will be the same color.

Chromosomes are in the shape of an X where one half is the original chromosome and the other half is the copy. These two copies are called chromatids. Color all chromatids and chromosomes blue.

3. Metaphase.  During this stage of mitosis, the chromosomes line up in the middle of the cell along the equator. Each chromosome attaches itself to a spindle fiber.  

4. Anaphase. During anaphase the chromatids are pulled apart by the spindle and move to opposite sides of the cell.

5. Telophase. Now that the chromosomes are separated, two nuclei are formed. The spindle fibers disappear; the chromosomes uncoil and become spaghetti-like chromatin again, and the nuclear membrane reappears.  Cytokinesis is where the cytoplasm splits into two daughter cells and usually occurs simultaneously with telophase.

2)

After prophase is complete, the cell enters prometaphase. During prometaphase, the nuclear membrane disintegrates and the mitotic spindle gains access to the chromosomes. During this phase, a protein structure called the kinetochore is associated with the centromere on each sister chromatid. Stringlike structures called microtubules grow out from the spindle and connect to the sister chromatids at their kinetochores; one microtubule from one side of the spindle attaches to one sister chromatid in each chromosome, and one microtubule from the other side of the spindle attaches to the other sister chromatid

3)

In contrast to eukaryotes, the DNA in prokaryotic cells is generally present in a single circular chromosome that is located in the cytoplasm. (Recall that prokaryotic cells do not possess a nucleus.) Prokaryotic chromosomes are less condensed than their eukaryotic counterparts and don't have easily identified features when viewed under a light microscope.

Eukaryotic chromosomes consist of repeated units of chromatin called nucleosomes, which were discovered by chemically digesting cellular nuclei and stripping away as much of the outer protein packaging from the DNA as possible. The chromatin that resisted digestion had the appearance of "beads on a string" in electron micrographs with the "beads" being nucleosomes positioned at intervals along the length of the DNA molecule

Nucleosomes are made up of double-stranded DNA that has complexed with small proteins called histones. The core particle of each nucleosome consists of eight histone molecules, two each of four different histone types: H2A, H2B, H3, and H4. The structure of histones has been strongly conserved across evolution, suggesting that their DNA packaging function is crucially important to all eukaryotic cells.

Histones carry positive charges and bind negatively charged DNA in a specific conformation. In particular, a segment of the DNA double helix wraps around each histone core particle a little less than twice. The exact length of the DNA segment associated with each histone core varies from species to species, but most such segments are approximately 150 base pairs in length. Furthermore, each histone molecule within the core particle has one end that sticks out from the particle. These ends are called N-terminal tails, and they play an important role in higher-order chromatin structure and gene expression.

4)

Prophase is where chromosomes become distinct masses under the microscope. The chromatin has condensed and will continue to condense until each chromosome is individually identifiable. It is usually possible to identify whether a cell is in early prophase:

There is a clear difference in the density of chromosomes over prophase, as the cell prepares to divide its contents. Meanwhile, the nuclear envelope is dissolving to allow the later events of mitosis to take place unhindered.

While the chromosomes continue to coil, the spindle fibres are growing. At first, they extend randomly in all directions, and the length of the fibres varies due to polymerization and depolymerization at both ends. However, polymerization at the centrosome, or (-) end, is relatively slow compared to the (+) end. The effect is that the (+) end extends rapidly into the cytoplasm. Some of the spindle fibres will come into contact with chromosomes, and in particular a protein complex around the centromere called the kinetochore. Once a spindle fibre connects to a kinetochore, it undergoes a change in stability. Once this happens, we can start to refer to spindle fibres more specifically by either calling them kinetochore fibres or polar fibres. These two different types of spindle fibres have unique roles when it comes to anaphase.

Despite the fact that DNA replication occurs prior to prophase, the two sister chromatids cannot be distinguished microscopically during early prophase. This is due to a phenomenon called relational coiling. As chromosomes condense during prophase, the relational coiling unwinds and each chromatid coils tightly with itself. Relational coiling disappears as prophase continues and the chromatids disengage to lie side by side.