cell division Flashcards
1
Q
What is the cell cycle?
A
The highly ordered sequence of events that takes place in a cell, resulting in division of the nucleus and the formation of two genetically identical daughter cells • 2 main phases in eukaryotic cells: interphase and mitotic phase
2
Q
What is interphase?
A
Growth period of the cell cycle, between cell divisions (mitotic phase). Consists of stages G1, S and G2. • The cell carries out all of it major functions e.g. producing enzymes or hormones, while actively preparing for cell division
3
Q
What happens during
interphase?
A
• DNA is replicated and checked for errors in the nucleus • Protein synthesis occurs in the cytoplasm • Mitochondria grow and divide, increasing in number in the cytoplasm • Chloroplasts grow and divide in plant and algal cell cytoplasm, increasing in number • Normal metabolic processes of the cell occur (some, including respiration, also occur throughout cell division)
4
Q
Describe the 3 stages of
interphase
A
G1 - 1st growth phase: proteins from which organelles are synthesised are produced, and organelles replicate. The cell increase in size S - Synthesis phase: DNA is replicated in the nucleus G2 - 2nd growth phase: cell continues to increase in size, energy stores are increased, and the duplicated DNA is checked for errors
5
Q
What is the mitotic phase?
A
Period of cell divine of the cell cycle. Consists of the stages mitosis (when the nucleus divides) and cytokinesis (the cytoplasm divides and two cells are produced)
6
Q
What is the G0 phase?
A
The phase when the cell leaves the
cell cycle either temporarily or
permanently
7
Q
List the reasons why a cell
may enter G0
A
• Differentiation - A cell that becomes differentiated can no longer divide; it carries out its function indefinitely and doesn’t enter the cell cycle again • Damaged DNA - A damaged cell can no longer divide and enters a period of permanent cell arrest (G0). Most normal cells only divide a limited number of times, eventually becoming senescent • Ageing - As you age the number of senescent cells in the body increases, linking with diseases e.g. cancer and arthritis
8
Q
Give an example of a cell
that can leave G0 after
entering
A
Lymphocytes (white blood cells) can
go back into the cell cycle and start
diving again in an immune response
9
Q
What are checkpoints and
why are they important?
A
Control mechanisms of the cell cycle They monitor and verify whether the processes at each phase of the cell cycle have been accurately completed before the cell is allowed to progress into the next phase They ensure the fidelity of cell division - that two identical daughter cells are created from the parent cell
10
Q
Describe what happens at each checkpoint in the cell cycle: 1. G1 checkpoint 2. G2 checkpoint 3. Spindle assembly/ metaphase checkpoint
A
1. End of G1 before entry into S. Check for: cell size, nutrients, growth factors, DNA damage 2. End of G2 before start of mitotic phase. Check for: cell size, DNA replication, DNA damage 3. At the metaphase point in mitosis. Check for: chromosome attachment to spindle
11
Q
What is the importance of
mitosis?
A
• Ensures that the 2 daughter cells produced are genetically identical to each other and to the parent cell • Needed for growth; replacement and repair of tissues in multicellular organisms; and asexual reproduction in eukaryotes
12
Q
Define the following:
- Chromosomes
- Chromatin
- Chromatids
- Centromere
A
1. Structures of condensed and coiled DNA in the form of chromatin. They become visible under the light microscope when cells are preparing to divide 2. Uncondensed DNA in a complex with histones 3. 2 identical copies of DNA (a chromosome) held together at a centromere 4. Region at which 2 chromatids are held together
13
Q
What happens before
mitosis?
A
• All of the DNA in the nucleus is replicated during interphase • Each DNA molecule is converted into 2 identical DNA molecules called chromatids • The 2 chromatids are joined at a centromere • Chromatids must be kept together during mitosis so that the can be segregated equally, 1 in each of the 2 new daughter cells
14
Q
Describe the stages of
mitosis:
A
1. Early Prophase • Chromatin fibres coil & condense to form chromosomes that take up stain to become visible under light microscope • Nucleolus disappears & nuclear membrane begins to break down • The centriole divides and the 2 new daughter centrioles move to opposite poles of the cell • Protein microtubules form a spindle fibres that will form a spindle between these centrioles • Spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell • By the end of prophase the nuclear envelope has disappeared 2. Metaphase • Chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate equator, and then held in position 3. Anaphase • Centromere of each pair of chromatids divide • Chromatids are separated and pulled to opposite poles of the cell by the shortening spindle fibres • Chromosomes assume a V shape 4. Telophase • Chromosomes reach the poles • New nuclear envelope forms around each set • Chromosomes start to uncoil and nucleolus is formed • Cytokinesis begins
15
Q
Describe cytokinesis in
animal cells
A
• Cleavage furrow forms around the middle of the cell • Cell-surface membrane pulled inwards by cytoskeleton until it is close enough to fuse around the middle forming 2 cells
16
Q
Describe cytokinesis in plant
cells
A
• Have cell walls so cleavage furrow can’t be formed • Vesicles from Golgi apparatus assemble in the same place as metaphase plate equator • Vesicles fuse with each other and cell surface membrane dividing the cell into 2 • New sections of cell wall then form along the new sections of membrane
17
Q
What is meiosis?
A
Form of cell division when the nucleus divides twice (meiosis I and meiosis II) resulting in a halving of the chromosome number and producing 4 haploid cells from one diploid cell. Reduction division: cell division resulting in the production of haploid cells from a diploid cell
18
Q
Why is meiosis needed?
A
To produce gametes that are haploid cells, so that when they fuse in fertilisation during sexual reproduction to produce a zygote, the zygote is a diploid cell
19
Q
What are homologous
chromosomes?
A
Matching pair of chromosomes, one inherited from each parent Alleles are different versions of the same gene As they have the same genes in the same positions, they will be the same length and size when they are visible in prophase. The centromeres will also be in the same positions
20
Q
What are the stages of
meiosis?
A
Meiosis I • The 1st division is the reduction division when the pairs of homologous chromes are separated into 2 cells • Each intermediate cell will only contain 1 full set of genes instead of 2, so the cells are haploid Meiosis II • The 2nd division is similar to mitosis • The pairs of chromatids present in each daughter cell are separated forming 2 more cells • 4 haploid daughter cells are produced in total
21
Q
Describe Prophase 1
A
• Chromosomes condense; nuclear envelope disintegrates, nucleolus disappears, spindle formation begins • The homologous chromosomes pair up, forming bivalents • Chromosomes are large DNA molecules and moving them through liquid cytoplasm as they are brought together results in chromatids entangling • This is called crossing over
22
Q
Describe Metaphase 1
A
• Pairs of homologous chromosomes still in their crossed over state assemble along the metaphase plate equator • Orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair • Independent assortment results in genetic variation
23
Q
Describe Anaphase 1
A
• Homologous chromosomes are pulled to the opposite poles and the chromatids stay joined together • Sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and rejoin -sometimes resulting in DNA exchange • When exchange occurs, recombinant chromatids are formed, with genes being exchanged between chromatids • Results in swapped areas of chromosome and allele shuffling
