Cell Continunity

Question 1

The ability for a cell to divide is essential to life. What is mitosis needed for

Answer 1

Cells to be replaced and for organisms to grow

Question 2

In between the process of one cell division and the next, the DNA is present in the nucleus as a diffuse granular mass called chromatin. This consists of what?

Answer 2

1. Densely stained heterochromatin where the DNA remains tightly wound and is strong and resistant to damage
2. Lightly stained euchromatin where the DNA is unwound to help promote protein synthesis.

Question 3

Mitosis: When a eukaryotic cell enters cell division the DNA becomes packaged into ______, visible under the ____ microscope. This ensures order and precise division of the DNA into two new daughter cells.

Answer 3

Chromosomes
Light

Question 4

What is the chromosome structure?

Answer 4

-The core is made up of 8 histones and a loop of DNA helix forms a nucleosome.

-Which further twists and coils to become a densely packed chromosome ready for cell division.

-At this stage it is composed of two chromatids joined at the centromere.

Question 5

What is the chromosome structure?

Answer 5

-The core is made up of 8 histones and a loop of DNA helix forms a nucleosome.

-Which further twists and coils to become a densely packed chromosome ready for cell division.

-At this stage it is composed of two chromatids joined at the centromere.

Question 6

The DNA is a long strand wound around proteins called histones. What is the function of these

Answer 6

This both protects and supports the
DNA.

Question 7

Stages of the cell cycle

Answer 7

Interphase is the non-dividing stage where the cell is metabolically active and the process consists of 3
stages
a. G1 is when the organelles and other cytoplasmic features are formed and the cell increases in size. It is only complete when the following steps have occurred
i. The cell reaches a certain size
ii. There are sufficient nutrients for the next stage
iii. Necessary growth factors are in place
iv. There is no DNA damage
At the end of G1 (a checkpoint): the cell will proceed to the next stage if all of the above have been met or if the steps have not been achieved then the cell enters a resting phase (G0). A cell can be in G0 for different periods of time eg nerve cells stay in G0 while epithelial cells rarely enter G0.

b. S is when the DNA replicates and the chromatids are formed (remember this is by semi conservative replication). Histones also form but the chromosomes are still not visible.

c. G2 is when the spindle proteins are synthesised and the DNA is checked for damage and repaired if necessary. There is another checkpoint at the end of G2.

Then mitosis; this where the nucleus divides into two daughters nuclei each with identical genetic information.

Cytokinesis is where the cytoplasm divides into two and two new daughter cells are formed. This process is different in plants and animals.

Question 8

Diploid chromosomes in humansdef

Answer 8

46 chromosomes in homologous pairs

Question 9

Fill in the blank: Diploid human cells have 46 chromosomes that are in pairs called ________ _______. These pairs are very similar in that they carry their genes in the same ________ but the genes may be in different forms aka ________eg the fur
colour may be the gene but the form or allele may be black, brown, white.

Answer 9

Homologus pairs

Sequence

Alleles

Question 10

What is a Karotype and karyogram

Answer 10

A karyotype is the number and appearance of chromosomes in a diploid cell. Humans have 46 chromosomes arranged in 23
pairs. A karyogram shows them arranged in homologous
pairs.

Question 11

Fill in the blanks: Mitosis is the production of ____ identical daughter cells from the original parent cell. They are in fact ____ of the parent and it is essential to replicate new cells with no DNA mistakes. There are ___ phases in mitosis

Answer 11

2

Clones

4

Question 12

Mitosis cell stage one and explain what happens vs meiosis stage one

Answer 12

Mitosis= Prophase (pro means forward as in go first). This is the longest phase of mitosis and the following
events occur:
a. The chromatin condenses and the chromosomes become visible as 2 chromatids joined at the
centromere
b. The nucleolus disappears
c. The centrioles (in animal cells) move to the poles and begin to form the spindle
d. The nuclear membrane breaks down

Meiosis= Prophase I
 chromosomes condense and become visible as 2 chromatids
 The nucleolus disappears
 The centrioles (in animal cells) move to the poles and begin to form the spindle
 The nuclear membrane breaks down
 Bivalents cross over ( see later notes

Question 13

Mitosis stage 2 vs meiosis stage 2

Answer 13

Mitosis in Metaphase (chromosomes move to the Middle).
a. Chromosomes move to the equator of the spindle and attach by the centromere
b. There is a checkpoint at this point to ensure the chromosomes have attached correctly to the spindle before anaphase.

Meiosis in Metaphase I:

 The bivalents line up along the equator of the cell
 The spindle fibres attach to the centromeres of each
chromosome

Question 14

Stage 3 mitosis vs meiosis

Answer 14

Mitosis stage 3:
Anaphase (chromatids move Apart)
a. The microtubules of the spindle are attached to the centromere of the chromosome. The centromere divides
b. As the microtubules contract they pull the chromatids apart until they reach the opposite ends or poles of the cell.

Meiosis stage 3:

 As the microtubules of the spindle contract the chromosomes are pulled to the opposite ends of the cell so splitting the homologous pair.
 This process ensures one chromosome from each pair is located in the new nuclei. It results in haploid cells.

Question 15

Stage 4 mitosis vs meiosis

Answer 15

Stage 4 mitosis:
Telophase (Two new nuclei formed)
a. The sister chromatids are at the opposite poles and are called chromosomes.
b. The chromosomes decondense and are no longer clearly visible.
c. The nucleolus reappears.
d. The nuclear membrane reforms.
e. Spindle disappears.

Stage 4 meiosis:
Telophase I
 Chromosomes consisting of 2 chromatids are pulled to opposite ends of the cell.
 The nuclear membrane reforms
 The chromosomes are no longer visible.

Question 16

(5th stage of meiosis and mitosis) Cytokinesis in mitosis vs in meiosis

Answer 16

Mitosis: In animal cells
The cell membrane pinches in as a cleavage furrow forms along the line of the equator and eventually causes the cell
to divide into two. (the furrow forms due to the contraction of a ring of actin and myosin protein)

In plant cells:
The Golgi body produces vesicles which lie along the equator. They contain pectin and hemicelluloses. The vesicles fuse together to form a cell plate stretching across
the cell. This forms the middle lamella and cellulose builds up on either side to form two new cell walls.

In meiosis:

 Each cell produces 2 daughter cells that are haploid. This is a total of 4 daughter cells from the original parent.
 May not always happen – a cell may continue into meiosis II

Question 17

Describe the final 6th stage of meiosis (mitosis doesn’t have this)

Answer 17

Meiosis II
 Prophase II happens immediately after meiosis I and the spindles are formed at right angles to that in meiosis I
 Pairs of chromatids (after crossing over) lie along equator in metaphase II
 Each chromosome is split into its 2 chromatids as they pulled to opposite poles in anaphase II
 Each new nuclei becomes enclosed in a nuclear membrane in telophase II
 Cytokinesis results in each daughter cell from meiosis I producing 2 new daughter cells.

Question 18

Meiosis is needed to both preserve the ______ ________ AND to ensure ______ _______

Answer 18

Chromosome number and genetic variation

Question 19

List the three ways genetic variation is achieved for meiosis (genetic variation is not considered in mitosis)

Answer 19

-crossing over
-Independent assortment
-Random fertilisation

Question 20

Explain genetic variation technique: Crossing over

Answer 20

Before meiosis begins ie during interphase, homologous chromosomes pair up and form bivalents. This results in four chromatids lying side by side.

Non sister chromatids sometimes cross over and break and exchange pieces of chromosomes with each other.

This always occurs during prophase I.
These points of break and exchange are called chiasma (pl chiasmata).

Since non sister chromatids will often carry different alleles, it leads to a
mix up of genetic information and sister chromatids are no longer identical. The chromatids involved in crossing over are now called recombinants.

Crossing over can hypothetically take place in every homologous pair of
chromosomes, thus vastly increasing the number of new allele
combinations in gametes.

Question 21

A genetic variation fill in the blank:

 The two chromosomes of a homologous pair have identical_____ but some _____ may differ between the two chromosomes.

 The two chromatids of each chromosome are identical before ______ ______ as they possess the same genes and the same
alleles.

 The longer the chromosome, the more ______ that can form.

Answer 21

Genes
Alleles
Grossing over
Chiasmata

Question 22

Explain genetic variation technique: independent assortment

Answer 22

During metaphase I bivalents are arranged randomly on the equator. In other words the homologous pairs lie in any arrangement of paternal and maternal chromosomes, so that the new nuclei receive a random mix of 23 maternal and paternal chromosomes.

Question 23

Explain genetic variation technique: random fertilisation

Answer 23

Random Fertilisation,The process of fertilisation increases the genetic variation of the offspring, as it is completely random which male gamete will fuse with which female gamete at fertilisation.

Question 24

Cancer involves _________ cell division and the production of a tumour. Hence it can be caused by problems with the _______ in the cell cycle, causing situations where cells repeatedly go through the cell cycle, despite there being no need.

There are many genes in humans which contribute to the regulation of the cell cycle, so that cells continue to divide in such a way as to meet the needs of the body. Many of these genes code for _______ which are involved in the checkpoints in some way.
For example one of the genes codes for a growth factor receptor. This receptor is on the cell surface membrane and when the growth factor is present, the cell can detect it and is allowed to proceed past the ___ checkpoint (assuming all other conditions are favourable).

Certain mutations in the gene can cause a _____ receptor to be produced, which is always in the ‘on’ state, whether the growth factor is present or not. The cell repeatedly goes through the whole cell cycle, dividing __________, when really it should be entering G0.

Answer 24

Uncontrolled
Checkpoints
Proteins
G1
Faulty
Continuously

Question 25

There can be genes responsible for detecting DNA damage at either the G1 or G2 checkpoint.

If significant DNA damage is discovered, the gene causes the cell to ‘self-destruct’. If this gene is mutated, then cells with damaged
DNA are allowed to_______, and this often results in a tumour. This gene is mutated in almost half of all cancers.

Answer 25

Divide

Question 26

__________ cell growth forms a large mass of cells called a tumour which can be benign or malignant.

A benign tumour _____ growing and becomes encapsulated in a ________ ________: none of the cells can break away and travel to other parts of the body.

A malignant tumour grows _______ and is the basis of cancer: cells can break free and travel to other parts of the body and grow to produce _______ tumours (metastases).

Cancer cells cannot enter ___ and the checkpoints that allow regulation of the cell cycle _____ _____

Answer 26

Uncontrolled
Stops
Fibrous sheath

Question 27

Many modern anticancer drugs work by disrupting the cell cycle and stopping _______. Chemotherapy drugs can affect normal cells as well as cancerous cells, as they stop cells ______. This leads to many unpleasant side
effects for patients.

Answer 27

Mitosis
Dividing

Question 28

Explain effects anti cancer drug Vincristine

Answer 28

binds to tubulin in microtubules- this inhibits the formation of microtubules. Therefore the spindle fibres are not formed. This means spindle fibres cannot contract to pull the chromatids apart.

Anaphase cannot take place. It is described as a mitotic poison.

Question 29

Explain effect of anti cancer drug that’s a antimetabolite e.g Flurouracil

Answer 29

Antimetabolites eg fluorouracil (5-FU) act on the S phase and prevent DNA synthesis. It inhibits the enzyme that makes nucleotides with the base thymine.

Question 30

Explain effects anti cancer drug cytoxan

Answer 30

Cytoxan prevents DNA unzipping, so preventing DNA replication.

Question 31

Vincristine not only affects cancerous cells; some of its side effects are due to its impact on the rapidly dividing epithelium in the ______ and in the _____ _____.

Vincristine is derived from the Madagascar periwinkle plant, as is another anticancer drug, vinblastine.

Answer 31

Intestine and bone marrow

Question 32

Where is chromosomes at prophase

Answer 32

Random mix in middle

Question 33

Where are chromosomes in metaphase

Answer 33

Aligned along equator in middle and are joined at there centromeres by spindle fibres

Question 34

Where are chromosomes in anaphase

Answer 34

Pulled apart, the contracting microtubules pull sister chromatids to opposite poles of the cell

Question 35

What do chromosomes look like in telophase

Answer 35

Two new nuclei are formed