Question

extension questions
model 3 - timing of dna replication
if the chromatin were condensed, replicated chromosomes would be visible. (end of s)
chromosomes are condensed. matching sister chromatids are connected by a centromere. (beginning of m)
if the chromatin were condensed, homologous chromosomes would be visible. (end of g1)

1. according to model 3, what term refers to loose dna inside of a nucleus?
2. during what part of the cell cycle is the dna in a cells nucleus replicated?
3. during what part of the cell cycle is the dna in a cell condensed into chromosomes?
4. replicated chromosomes are often illustrated as an x shape to match how they look in real life just before cell division.

a. according to model 3, which of the following diagrams correctly show an original set of homologous chromosomes (grey) and their sister chromatids (black) - the replicated portion?
b. what structure holds the two sister chromatids together as they prepare for cell division?

Explanation:

1. Chromatin is the term for loose DNA inside the nucleus. It is the un - condensed form of DNA present during most of the cell cycle.
2. DNA replication occurs during the S (synthesis) phase of the cell cycle. This is when the cell duplicates its genetic material.
3. DNA condenses into chromosomes during the M (mitosis) phase, specifically at the beginning of mitosis when the chromatin fibers coil and condense.

20a. The correct diagram would show homologous chromosomes (same size and shape, different colors to represent maternal and paternal origin) with sister chromatids (identical copies, same color) attached at the centromere. Without seeing the actual diagrams, the general concept is that homologous chromosomes come in pairs and sister chromatids are the replicated parts of each chromosome.
20b. The centromere is the structure that holds the two sister chromatids together as they prepare for cell division. It is a specialized region of the chromosome.

Answer:

1. Chromatin
2. S phase
3. M phase

20a. (No specific answer given as diagrams not described in detail)
20b. Centromere