Chromosomes and Meiosis

Question 1

What are chromosomes?

Answer 1

• long, thread-like structures
• that form part of the chromatin network in the nuclei of cells

Question 2

What are chromosomes made up of?

Answer 2

• a strand of DNA wound around proteins called histones

Question 3

How many chromosomes are in the somatic cells of diploid organisms?

Answer 3

• the number of chromosomes in each cell is the same

Question 4

What are the chromosomes of somatic cells made up of?

Answer 4

• two sets
• one chromosome of each pair comes from the mother and one comes from the father
• therefore diploid cells

Question 5

What is a homologous pair formed?

Answer 5

• when a paternal chromosome and a maternal chromosome pair up

Question 6

What does the DNA of each chromosome do?

Answer 6

• replicated to form 2 identical threads or chromatids joined by a centromere

Question 7

When does DNA replication occur?

Answer 7

• in the interphase of a cell cycle

Question 8

When do chromosomal threads become visible?

Answer 8

• only when they shorten and thicken as a cell divides

Question 9

What happens to the number of chromosomes in meiosis?

Answer 9

• the number of chromosomes is reduced from two sets (2n) in the parent cell
• to one set (n) in each of the daughter cells formed
• i.e. the number of chromosome is halved

Question 10

What is meiosis?

Answer 10

• cell division that takes place in the reproductive organs of both plants and animals
• to produce gametes in animals
• and spores in plants

Question 11

What are the characteristics of the gametes/spores formed in meiosis?

Answer 11

• they are called haploid
• as they only have one set of chromosomes
• i.e. one chromosome from each homologous pair

Question 12

Where does meiosis take place in animals?

Answer 12

• the testes and ovaries

Question 13

What is the formation of sperm cells in the testes called?

Answer 13

• spermatogenesis

Question 14

What is the formation of egg cells in the ovaries called?

Answer 14

• oogenesis

Question 15

Where does meiosis take place in plants?

Answer 15

• in sporangia

Question 16

What are microsporangia?

Answer 16

• the pollen sacs in the male anthers

Question 17

What are megasporangia?

Answer 17

• the ovules in the female ovaries

Question 18

What does abnormal meiosis result in?

Answer 18

• chromosome abnormalities

Question 19

What is a way in which abnormal meiosis can occur?

Answer 19

• chromosome non-disjunction

Question 20

When does chromosome non-disjunction occur?

Answer 20

• when chromosomes fail to separate correctly during meiosis

Question 21

What happens if chromosome non-disjunction occurs in the sex chromosomes?

Answer 21

• fertilisation involving one of these abnormal gametes will result in a zygote with either an extra or missing chromosome
• a condition known as aneuploidy

Question 22

What is aneuploidy?

Answer 22

• a condition wherein chromosomes fail to separate correctly during meiosis
• resulting in a zygote with either an extra or a missing chromosome

Question 23

What is a diagnosis?

Answer 23

• the identification of the nature of an event

Question 24

What is a prognosis?

Answer 24

• the prediction of how the event will develop

Question 25

What are some examples of aneuploidy?

Answer 25

- klinefelter's syndrome - down syndrome

Question 26

When does klinefelter's syndrome occur?

Answer 26

- when a boy is born with an extra copy of the x chromosome - when the sex chromosomes in the egg (or very rarely in the sperm) split unevenly

Question 27

What are some of the possible symptoms of klinefelter's syndrome?

Answer 27

- sparse body hair - enlarged breasts - small testicles and penis - not very deep voices - infertility

Question 28

When does down syndrome occur?

Answer 28

- when children are born with an extra copy of chromosome number 21

Question 29

How does down syndrome come about?

Answer 29

- during oogenesis, the 2 number 21 chromosomes do not separate properly during anaphase 1 - both go into 1 daughter cell instead of 1 into each of the 2 cells formed

Question 30

What are some down syndrome characteristics?

Answer 30

- varying degrees of mental retardation - distinctive flattened facial features with slanting eyes - often heart defects

Question 31

What is a karyotype?

Answer 31

- a set of chromosomes in a cell

Question 32

What do karyotypes show?

Answer 32

- the number, size and shape of the chromosomes during metaphase of mitosis

Question 33

What are non-sex chromosomes called?

Answer 33

- autosomes

Question 34

What are sex chromosomes called?

Answer 34

- gonosomes

Question 35

Why are karyotypes useful?

Answer 35

- can show whether a cell comes from a male or a female - can show abnormal chromosomes