Cell cycle, mitosis & meiosis

Question 1

What is a somatic cell?

Answer 1

Any cell of a multicellular organism that is not a gamete

Question 2

What is the cell cycle?

Answer 2

A highly ordered sequence of events that takes place in a cell, resulting in division of the cell, and the formation of two genetically identical daughter cells

Question 3

What are the 3 stages of the cell cycle?

Answer 3

• Interphase
• Mitosis => nuclear division
• Cytokinesis => cell division

Question 4

What are the three main stages of interphase?

Answer 4

• G₁
• S
• G₂

Question 5

What occurs during the G₁ stage of interphase?

Answer 5

• It is the first growth phase
• proteins from which organelles are synthesised are produced
• Organelles replicate
• The cell increases in size
• Intake of nutrients

Question 6

What occurs in the S stage of interphase?

Answer 6

• This is the synthesis phase
• DNA is replicated in the nucleus
• Histones are synthesised

Question 7

What occurs during the G₂ stage of interphase?

Answer 7

• This is the second growth phase
• Organelles duplicate
• Production of proteins required for mitosis + cytokinesis
• The cell continues to increase in size
• Energy stores are increased
• The duplicated DNA is checked for errors

Question 8

What is the G₀ stage?

Answer 8

It is the name given to the ohase when the cell leaves the cycle, either temporarily or permanently

Question 9

What are the reasons for a cell entering G₀?

Answer 9

• Cell differentiation and specialisation => a cell that becomes specialised to carry out a specific function loses the ability to divide. It will carry out this function indefinetly and will remain in G₀ for the rest of their lifetime
• Apoptosis => programmed cell death
• The DNA of a cell may be damaged => the cell is no longer viable. A damaged cell can no longer divide and enters a period of permanent cell arrest (G₀). The majority of normal cells only divide a limited number of times and eventually become senscent
• Sencscence => ageing of cells
• A resting phase => a period of inactivity or dormancy before going back to G₁. These are referred to as quiescent cells. Cells may renter G₁ due to external stimuli

Question 10

What are checkpoints? And what are their functions

Answer 10

• The control mechanisms of the cell cycle
• They monitor and verify whether the processes at each pahse of the cell cycle have been accurately completed before the cell is allowed to progress into the next phase

Question 11

What checkpoints are in the cell cycle?

Answer 11

• G₁ checkpoint
• G₂ checkpoint
• Spindle fibres assembly checkpoint (also called metaphase check point)

Question 12

What is the G₁ checkpoint?

Answer 12

• This checkpoint is found at the end of the G₁ phase, before entry into S phase
• If the cell satisfies the requirements of this checkpoint it is triggered to begin DNA replication
• If not, it enters a resting state (G₀)

Question 13

What is the G₂ checkpoint?

Answer 13

• This checkpoint is at the end of the G₂ phase, before the start of the mitotic phase
• In order for this checkpoint to be passed, te cell has to check a number of factors, including whether the DNA has been replicated without error
• If this checkpoint is passed, the cell initiates the molecular processes that signal the beginning of mitosis

Question 14

What is the spindle fibres assembly checkpoint?

Answer 14

• This checkpoint is at the point in mitosis where all the chromosomes should be attached to spindle and have aligned
• Mitosis cannot proceed until this checkpoint is passed

Question 15

What is the region where two chromatids are joined together?

Answer 15

Centromere

Question 16

What is a chromosome?

Answer 16

• A molecule of DNA (present as chromatin in interphase)
• Chromosomes condense and appear in mitosis and meiosis

Question 17

What is a homologous chromosome?

Answer 17

Homologous chromosomes are a pair of chromosomes, one from each parent, which have:
- A similar length
- The same genes at the same loci (positions)
- Centromeres in the same position

Question 18

What are the four stages of mitosis?

Answer 18

• Prophase
• Metaphase
• Anaphase
• Telophase

Question 19

What occurs in prophase of mitosis?

Answer 19

• Chromatin fibres begin to coil and condense to form chromosomes
• The nucleolus disappears
• The nuclear envelope degrades
• Spindle fibres (formed from microtubules) are fully formed
• Two centrioles have migrated to opposite poles and formed centrosomes
• Chromosomes appear and each has a pair of identical chromatids
• Centromeres attach the sister chromatids to the spindle fibres

Question 20

What occurs in metaphase of mitosis?

Answer 20

• Spindle fibres start to emerge from the centrosome at each pole
• Chromsomes line up at the equator
• Centromeres attach the sister chromatids to the spindle fibres

Question 21

What occurs in anaphase of mitosis?

Answer 21

Identical sister chromatids are separated by being pulled apart to opposite poles by spindle fibres depolymerising (shortening)

Question 22

What occurs in telophase of mitosis?

Answer 22

• Chromosomes disperse (decondense) and chromatin reforms
• Nuclear envelope reforms
• Nucleolus is formed

Question 23

What is cytokinesis? And how does it differ in plant and animal cells?

Answer 23

• It is the actual division of the cell into two seperate cells and it begins during telophase
• In animal cells, cytokinesis involves the equator region invaginating
• In plant cells, cytokinesis involves the formation of a cell wall at the equator

Question 24

How do you calculate the mitotic index?

Answer 24

mitotic index = the number of cells in mitosis/total number of cells

Question 25

What is a haploid cell?

Answer 25

A cell containing a single set of chromosomes (=> unpaired chromosomes)

Question 26

What is a diploid cell?

Answer 26

A cell containing pairs of homologous chromosomes, one from each parent

Question 27

What are homologous chromosomes?

Answer 27

Pairs of chromsomes in a diploid cell, one from each parent, of similar size and containing the same genes (which may have alternative alleles)

Question 28

What is a gamete?

Answer 28

A haploid cell required for sexual reproduction (sperm/pollen for moles and egg/ovum for females)

Question 29

What is fertilisation?

Answer 29

A fusion of female and male gametes to form a zygote

Question 30

What is a zygote?

Answer 30

A diploid cell formed as a result of the fusion of two halpoid gametes

Question 31

What are the stages of meiosis?

Answer 31

Meiosis I: - Prophase I - Metaphase I - Anaphase I - Telophase I Meiosis II: - Prophase II - Metaphase II - Anaphase II - Telophase II

Question 32

What occurs in prophase I of meiosis?

Answer 32

- Chromatin condense, the nuclear envelope disintegrates, the nucleolus disappears and spindle fibre formation begins - The homologous chromosomes pair up, forming bivalents - Chromosomes are large molecules of DNA and moving them through the liquid cytoplasm as they are brought together results in the chromatids entangling - This is called crossing over - Sections of DNA on 'sister' chromatid, which become entangled break off and rejoin resulting in an exchange of DNA - The points where the chromatids break and rejoin are called the chiasmata - Exchange occurs forming recombinant chromatids and sister chromatids are no longer identical resulting in genetic variation due to a new combination of alleles

Question 33

What occurs in metaphase I of meiosis?

Answer 33

- The bivalents line up on the equator - The orientation of the bivalents is random and independent of other bivalents - This is called independent assortment, which results in different combinations of alleles

Question 34

What occurs in anaphase I of meiosis?

Answer 34

- Homologous chromosomes are pulled apart to opposite poles

Question 35

What occurs in telophase I of meiosis?

Answer 35

- Chromosomes assemble at each pole and the nuclear envelope reforms - Chromosomes decondense into chromatin and nucleolus becomes visible again - Cell undergoes cytokinesis and divides into two cells - Reduction division => cell goes from diploid to haploid

Question 36

What occurs in prophase II of meiosis?

Answer 36

- Chromosomes, which still consist of two chromatids, become visible again - The nuclear envelope breaks down, nucleolus disappears and spindle fibres form

Question 37

What occurs in metaphase II of meiosis?

Answer 37

- Individual chromosomes line up on the equator of the cell - There is an independent assortment of non identical sister chromosomes producing more genetic variation

Question 38

What occurs in anaphase II of meiosis?

Answer 38

- Chromatids of the individual chromosomes are pulled apart to opposite poles - Spindle fibres depolymerise

Question 39

What occurs in telophase II of meiosis?

Answer 39

- Chromatids assemble at the poles and decondense to form chromatin - Nuclear envelope reforms and nucleolus becomes visible - Cytokinesis results in cell division of the cells forming four genetically different daughter cells

Question 40

Draw the alternation of generations.

Answer 40

Answer on revision card