DNA, DNA Replication.Chromosomes, the cell cycle and mitosis

Question 1

What is the basic building block of DNA?

Answer 1

a nucleotide.

Question 2

describe the structure of a DNA nucleotide.

Answer 2

a phosphate group, a pentose sugar (deoxyribose), and a base pair.

Question 3

describe how a DNA strand is formed.

Answer 3

Formed by two or more mono nucleotides, being joined together by a condensation reaction, thus forming a phosphodiester bond between pentose sugar and phosphate.

Question 4

what are the central bases joined together by in DNA?

Answer 4

hydrogen bonds.

Question 5

describe how the bases pair with each other.

Answer 5

adenine with thymine
Guanine with cytosine.

Question 6

describe all the structures of DNA, and relate this to its function.

Answer 6

1. base sequence + stores genetic information.
2. Large = stores massive amount of genetic information.
3. Helix : Allows it to be compact.
   4.sugar phosphate backbone : provides stability and protection, thus reducing mutations.
4. hydrogen bonds between bases : break and reform, in order to allow replication and Protein synthesis.

Question 7

identify all the similarities between DNA in prokaryotes and DNA in eukaryotes.

Answer 7

1.The nucleotide structure is identical.
2. the nucleotides are joined together by phosphodiester bonds.

Question 8

identify all the differences between DNA in prokaryotes and DNA in eukaryotes.

Answer 8

1.Eukaryotes contain both introns and exons, whereas prokaryotes only exons.
2. eukaryotic DNA is long and linear, whereas prokaryotic DNA is short and circular.
3. eukaryotic DNA is associated with histones, whereas prokaryotic DNA isn’t.

Question 9

Describe the structure of a chromosome.

Answer 9

The DNA first condenses into chromatin, by wrapping tightly around the histones, thus enabling it to fit in the nucleus.

The chromatin then folds together, to fold a chromatid.

At the end of each chromatid, there are telomeres, which prevent DNA from degrading.

The chromatids are then joined together by a centromere.

Question 10

Describe binary fission in prokaryotes.

Answer 10

the prokaryotic singular, short and circular DNA is replicated. Any plasmids present in the cytoplasm are also replicated.

The cytoplasm then divides into two identical cells. these two may have different number of plasmids, but have the same copy of circular DNA.

All of these 3 cells, are genetically identical.

Question 11

Where do all the cells in the human body derive from?

Answer 11

they all derive from a single zygote.

Question 12

What are the 3 phases of the cell cycle?

Answer 12

Interphase

mitosis

cytokenesis.

Question 13

describe interphase.

Answer 13

during interphase, the cell replicates its DNA and organelles.
-this doubles the mass of DNA and the cell overall.
The cell also checks for any replicated DNA that has errors, and the further repairs them.

interphase accounts for 90% of the cell cycle.

Question 14

describe mitosis ( broadly)

Answer 14

This is when the nucleus divides into two.
the mass of the DNA and the mass of the cell overall, remains unchanged.

Question 15

describe and explain all the roles of mitosis.

Answer 15

growth: from zygote, to multicellular organisms.
repair: damaged cells, can be quickly replaced by genetically identical cells.
reproduction: enables asexual reproduction.
immune system: enables the production of large number of white blood cells.

Question 16

Describe how to calculate the mitotic index.

Answer 16

the proportion of cells that are undergoing mitosis.
mitotic index : number of cells undergoing mitosis divided by total number of cells in a population.

Question 17

What are the 4 subphases of mitosis? In order.

Answer 17

Prophase then metaphase, then anaphase, then telophase.

Question 18

describe prophase

Answer 18

Chromatin condenses into compact chromatids., which unlike chromatin, are visible under microscope.

These are arranged into sister chromatids, joined together by centromeres.

The nucleolus disappears and the nuclear envelope breaks down, therefore the chromosomes float free in the cytoplasm.

protein microtubules, called spindle fibres, form at opposite poles of the cell, at centrosomes.

Plant cells don’t have centrosomes, however, they still have spindle fibres.

Question 19

describe metaphase.

Answer 19

spindle fibres, attach to the centromeres of the chromosomes,

while the chromosomes line up at the equator of the cell.

each sister chromatid is attached to an opposite pole.

Question 20

Describe anaphase.

Answer 20

the spindle fibres shorten, causing the sister chromatids to be pulled towards opposite ends of the cell.

Question 21

describe telophase.

Answer 21

The chromatids reach the cell poles, and decondense back into chromatin.

Nuclear envelopes, and nucleolar reform, and the spindle fibres break down.

At the end of telophase, there is a fully formed identical nucleus, at each pole of the parents cell

They are then ready to be divided by cytokenisis

Question 22

describe cytokenesis.

Answer 22

This is when the cell divides into two, thus leaving two genetically identical daughter cells.

Question 23

What determines the rate of mitosis? and explain what happens if this is not controlled.

Answer 23

the rate of mitosis is limited by certain genes.

If these genes mutate, cell division can become uncontrolled, thus leading to the development of a tumour, and possibly cancer.