Chapter 6.1 cell Cycle

Question 1

What are three stages of cell cycle? (Or two)

Answer 1

First is interphase, where there is long periods of growth

Then is the mitotic division stage consisting of mitosis and cytokinesis

Question 2

What happens in interphase roughly (3)

- why is it referred to as a resting phase but what actually is it

Answer 2

• Long periods of growth and a cell spends most of the cell cycle here
• Preparation for division
• normal functions like respiration still carried out

-2)people refer to interphase as the resting phase, however this is NOT TRUE.
: it actually is a very active phase of cell cycle as cell carries out all replication including producing enzymes hormones etc, whilst actively preparing for cell division

Question 3

What happens in first stage

Answer 3

G1- First growth phase ,

• proteins that make organelles are synthesised
• ALL organelles APART FROM CHROMOSOME then duplicate (including mitochondria / chloroplasts )
• CELL increases in size

Question 4

What happens in second stage and what called

Answer 4

S Phase (synthesis of DNA )
- here DNA in chromosomes is fully replicated in the nucleus

Question 5

What happens in last phase (what called)

Answer 5

G2= second growth phase

• here cell continues to increase in size
• energy stores increase (ATP)
• proteins needed for cell division made

Then G2 check happens

Question 6

What is full order of Interphase?

Answer 6

G1
G1 CHECKPOINT 
S Phase / G0 Phase
G2 
G2 checkpoint

Question 7

When are checks for interphase and what is checked?

What does it do

Answer 7

G1 Check, at the end of G1 and bringing of S

• checks for cell size, nutrients growth factors and DNA damage
• proteins including helicase etc for s phase

2) G2 check at the end of G2 before mitosis
- checks for whether DNA HAS BEEN REPLICATED CORRECTLY
- checks for any damage to DNA and duplicated
- fixes these makes repairs
- checks for cell size

If check not complete not allowed to go into next stage

Question 8

What is the G0 phase
Why do they enter here (2)
What does SENSECENT ! mean and how is this linked with old age ?(also normal cells how many times they can divide

What cells can return from G0

Answer 8

Phase given to the stage where the cell leaves the cell cycle , temporarily or forever,

1) - if a cell becomes specialised it has differentiated to perform a specific function where it does this indefinitely and thus NO LONGER DIVIDES
- the DNA of a cell may have become damaged so it is no longer viable . The cell is permanently under cell arrest in G0 and NO LONGER DIVIDES (still alive tho )

2) senescence is the inability to divide . the majority of cells only able to divide a few times and end up becoming senescent . As you grow old the number of this INCREASES and this is linked with diseases
3) cells like liver few times and lymphocytes

Question 9

What is the ACTUAL definition of mitosis (which sets it aside from cytokinesis )

Answer 9

The division of the nucleus , (rather than physical division)

Question 10

When and why does mitosis happen

Does mitosis happen In prokaryotes ?

Answer 10

Mitosis happens when the two daughter cells need to be genetically identical

• thus in the case of growth , repairing tissue and replacement of cells
• asexual reproduction to give genetically identical offspring

2) NO THIS IS BINARY FISSION

Question 11

What is the structure of DNA MOLECULES (chromatin how) before going into mitosis and as a result of interphase ?

Why held at centromere

Answer 11

By now DNA has replicated
1) IN INTERPHASE dna combines with proteins (cohesion ) and histone to make a Long thin molecule called chromatin which is loosely coiled and csn’t see under microscope

2) all the chromatin now converts into two strands called CHROMATIDS that are joined together in the middle at the CENTROMERE (one chromosome made up of two adjacent chromatids)
- two adjacent chromatids of the same CHROMOSOME are called sister Chromatids

2) it is essential they held in the middle so that they can be manoveured around and segregated equally at the end

Question 12

What happens in early prophase?

Answer 12

2) CHROMATIN MOLECULES NOW CONDENSE AND APPEAR AS CHROMOSOMES!
- the Nucleolus (responsible for RNA synthesis ) breaks down
- the NUCLEAR MEMBRANE begins to breakdown , leaving chromosomes free in cytoplasm
- mitotic spindle fibre begins to be made , and centrioles move to opposite ends of the cell

Question 13

What happens in late prophase / pro metaphase

Answer 13

Here microtubules invade the nuclear area as they extend to each centromere , beginning to pull

• by the end of prophase the nuclear membrane is fully broken down
• chromosomes condense even more (now can take up stain and be seen on microscope)

Question 14

What happens at metaphase?

WHAT ELSE ? (Why this important to happen)

Answer 14

Chromosomes (Esch With two chromatids) are moved by the spindle fibre to the centre of the cell , called the METAPHASE PLATE (equator and equidistant from cell poles) with the centromeres at the centre

+ THEN METAPHASE SPINDLE FIBRE CHECKPOINT HAPPENS

• here it is checked every single chromosomes is attached to each spindle fibre correctly.
• this important because if not you could get an uneven distribution of chromosomes in each resulting cell Causing complications

Question 15

What happens at anaphase

Answer 15

• the centromeres holding pairs of chromatids now divide
• this allows the spindle fibres to pull on the divided centromeres by CONTRACTING
• thus pulling sister chromatids to opposite pole of the cell (gives v shape)

Now chromosomes are at each end of the cell

Question 16

What happens in telophase?

Answer 16

• the sister chromatids are now called chromosomes
• the nuclear envelope begins to reform around both sets of chromosomes , eventually forming the Nucleolus again.
• the chromosomes become less condensed and uncoil

• cytokinesis begins with the pinching in of the the two cells

Question 17

Recap what happens in mitosis

Learn what’s in bold

Answer 17

1) early prophase, the DNA arranged in chromatin begins to condense, Nucleolus breaks down and nuclear membrane begins to break down. Here mitotic spindle fibre begins to be made and duplicated centrioles move to opposite ends of the cell
1b)in early metaphase pro metaphase, the spindle fibre now extends to each centromere, begins to pull. Nuclear envelope completely fragmented, and chromatin chromosomes condense even more (can be seen )
2) metaphase, the spindle fibre moves each pair of chromatids , so that they line up on the middle (equator), called metaphase plate.
- THEN METAPHASE CHECKPOINT HAPPENS, only after this will they let it continue
3)in anaphase, the spindle fibre moves Each pair of chromatids to opposite ends of the cell, this is done by first the centromere dividing snd then spindle fibre contracting whilst attached to it .
4) telophase, the chromatids are now chromosomes . Opposite sets of chromosomes are now surrounded by a nuclear envelope reforming, and Nucleolus giving two nuclei. At this point mitosis is complete.
- ALSO CHROMOSOMES BEGIN TO UNCOIL AND BECOME LOOSE AGAIN
The act of cytokinesis begins as the cell membrane begins to pinch inwards

Question 18

How does cytokinesis happen in animals REMEMBER

Answer 18

A cleavage furrow forms around the middle of the cell.

• cel, surface membrane is pulled inwards by the cytoskeleton until it is close enough to FUSE around the middle
• this forms two cells

Question 19

How does cytokinesis happen in plant cells

What would happen if cell wall made first

Answer 19

• as plant cells have a cell wall, they cannot pinch and split by using a cleavage furrow
• instead they have to use vesicles from the Golgi apparatus
• these assemble at the metaphase plate again and FUSE with each other and the cell surface membrane to form a cell plate which splits the two cells
• then cell walls form along this section

2. Then osmosis would cause lysis

Question 20

How to investigate mitosis with root tips

Why use root cells good idea

Answer 20

• treat the root tips with hydrochloric acid
• spread them apart just slides
• stain and squash and then view

2) it’s a source of meristem tissue which is a place that allows for dividing cells undergoing mitosis . Plants are easy to source and easy to prepare for microscopy too