Lecture 27: Chromosomes And Mitosis

Question 1

DNA is packed into chromosome. Explain

Answer 1

The double stranded DNA is highly compacted when it is packed into chromosomes.
-The DNA helix first wi do around histones to from nucleosomes.
-Nucleosomes coil upon themselves, condense and compact to form chromatin.
-chromatin is a combination of DNA and protein molecules
-each chromosome is one long DNA strand bearing thousands of genes
-genetic info is stored as sequence of nucleotide bases on DNA.
Chromosomes are only visible in the nucleus when condensed preparing for division of the nucleus.
The rest of the time chromosome exist as a long, thin mass of very long thin fibres. This is called chromatin

Question 2

Preparation for cell division

Answer 2

Before the eukaryotic cell divides, it duplicates all of its chromosomes.

• the DNA of each molecule is copied and new protein molecules attached
• the result is that now each chromosome consists of 2 copies called sister chromatids
• the two chromatids are joined at the centromere

Question 3

Mitosis- divides somatic cells in eukaryotes

What are somatic cells

Answer 3

Somatic cells are all cells of an organism except gametes (not eggs or sperm)
Eg skin, liver, muscle, fat
In somatic cells the
The chromosomes exist in pairs, one from mother and one from father
They are diploid- 2 copies of each chromosome

Question 4

Chromosomes at mitosis

Answer 4

• After replication of DNA during the S phase chromosomes consist of 2 copies joined at the centromere.
• the movement of the chromosomes depends in the mitosis spindle, which guides the seperation of the 2 sets of chromosomes
• these 2 identical strands are called sister chromatids
• the two chromosomes of such a pair are called homologous chromosomes (they both carry the same series of genes)

Question 5

Describe the stages of mitosis

Answer 5

1. Prophase:
   - the chromosomes condense to become 2 discrete chromatids joined at the centromere
   - the nuclear membrane disintegrates
2. Metaphase
   - the mitotoc spindle forms from micro tubules
   - chromosomes consisting of 2 chromatids attach to the spindle at the centromere
   - chromosomes move to the centre of the equator under the control of the mitotic spindle
3. Anaphase
   - the chromosomes divide s hat each chromatid has its own centromere
   - chromosomes move to the opposite poles of the cells pulls by their centromeres
4. Telophase
   - the chromosomes uncoiled
   - the nuclear membrane reforms
   - the cytoplasm divides by cytokinesis (membrane pinches in)
   - the cell membrane of the 2 daughter cells reforms

Question 6

Results of mitosis

Answer 6

Two diploid cells are formed

• each has the same number of chromosomes as the original cell
• there has been one division of the nucleus and one division of the cell

Question 7

Explain the clinical use of a karyotype

Answer 7

A karyotype is the arrangement of a cells chromosomes from largest to smallest.

• genetic testing
• karyotypes are used to detect gross genetic defects- missing or duplicate chromosomes
• also can be used to determine se
• x and Y chromosomes are easily visible

Question 8

What does diploid mean

Answer 8

Two copies of the chromosome