Meiosis & genetics (meiosis & variation) Part 1 - (Week 8)

Question 1

What is a life cycle?

Answer 1

The generation-to-generation sequence of stages in the reproductive history of an organism
(It starts at the conception of an organism until it produces its own organism)

Question 2

How many chromosomes do humans have in each of their somatic cells (all cells except sperm & ovum)?

Answer 2

Total of 46 chromosomes per somatic cell

23 homologous pairs

Question 3

Where are each of the chromosomes of a homologous pair from?

Answer 3

One inherited from each parent

23 homologous pairs so can be seen as two sets of 23, one maternal set, one paternal

Question 4

What do these homologous chromosome pairs do?

Answer 4

They carry genes that control the same inherited characters

Question 5

What is the result of meiosis?

Answer 5

The chromosome number is halved for gametes that undergo this process

Question 6

How many chromosomes are found in a human sperm or ova?

Answer 6

23 different chromosomes (haploid set - not paired)

One from each homologous pair

Question 7

What is the difference between meiosis & mitosis?

Answer 7

• In meiosis, there are two consecutive cell divisions, meiosis I & meiosis II, which results in 4 daughter cells
• Each final daughter cell has only half as many chromosomes as the parent cell
• Daughter cells produced are not identical to one another & differ from the parent cells

Question 8

Give the 4 phases of meiosis

Answer 8

• Prophase
• Metaphase
• Anaphase
• Telophase
  (PMAT)

Question 9

What needs to happen before meiosis occurs?

Answer 9

Interphase:

• The chromosomes are replicated to form sister chromatids - these are joined at the centromere
• The single centrosome is also replicated

Question 10

Describe what happens in prophase I

Answer 10

• The chromosomes condense & homologous chromosomes pair up to form tetrads
• Synapsis occurs
• Chiasmata occurs at several sites
• Spindle forms from each centrosome & spindle fibres attached to kinetochores on the chromosomes begin to move the tetrads

Question 11

What is synapsis?

Answer 11

Special proteins attach homologous chromosomes tightly together

Question 12

What is chiasmata?

Answer 12

When the chromatids of homologous chromosomes are crossed - which allows for segments of chromosomes to be traded (genetic rearrangement)

Question 13

Describe what happens in metaphase I

Answer 13

• The tetrads are all arranged at the metaphase plate (equator of cell)
• Microtubules from one pole are attached to the kinetochore of one chromosomes of each tetrad, while those from other pole are attached to the other

Question 14

Describe what happens in anaphase I

Answer 14

The homologous chromosomes separate & are pulled toward opposite poles

Question 15

Describe what happens in telophase I

Answer 15

• Movement of homologous chromosomes continues until there is a haploid set at each pole (each chromosome consists of linked sister chromatids)
• Cytokinesis usually occurs simultaneously (cell divides into 2 in same way as mitosis)
• In some species, nuclei may reform, but there is no further replication of chromosomes

Question 16

What process is meiosis II similar to?

Answer 16

Mitosis

Question 17

Describe what happens during prophase II

Answer 17

• A spindle apparatus forms, attaches to kinetochores of each of the sister chromatids & moves them around
• Spindle fibres form one pole, attach to the kinetochore of one sister chromatid & those of the other pole to the other sister chromatid

Question 18

Describe what happens in metaphase II

Answer 18

• The sister chromatids are arranged at the metaphase plate

- The kinetochores of sister chromatids face opposite poles

Question 19

Describe what happens in anaphase II

Answer 19

The centromeres of sister chromatids separate & the now separate sisters travel toward opposite poles

Question 20

Describe what happens in telophase II

Answer 20

• Separated sister chromatids arrive at opposite poles
• Nuclei form around the chromatids
• Cytokinesis separates the cytoplasm
• At the end of meiosis, there are four haploid daughter cells

Question 21

Synapsis occurs only in meiosis, not in mitosis. True or false?

Answer 21

True - there is no synapsis in mitosis

Question 22

What type of cells are produced in meiosis?

Answer 22

4 genetically different haploid cells (half the no. of chromosomes)

Question 23

What is the difference between meiosis I & II?

Answer 23

• The first division, meiosis I, separates homologous chromosomes
• The second, meiosis II, separates sister chromatids