Meiosis

Question 1

Why does meiosis happen?

Answer 1

-When gametes fuse at fertilisation to form a zygote, the diploid number is restored which enables a constant chromosome number is maintained from generation to generation
-Introduces variation into the genetic material

Question 2

What is monosomy?

Answer 2

A sex cell which is missing one chromosome

Question 3

What is polysomy?

Answer 3

A condition where there are more chromosomes than required

Question 4

What is aneuploidy?

Answer 4

An abnormal number of chromosomes

Question 5

What is turners syndrome?

Answer 5

A genetic condition
-Only affects females
-Only have one X chromosome
-They are missing a Y chromosome
-Caused by monosomy

Question 6

What is down syndrome?

Answer 6

A genetic condition
-Caused by having an extra chromosome 21
-Caused by polysomy

Question 7

What is translocation?

Answer 7

-Take place during meiosis
-The chromosomes can be balanced or unbalanced
-They can change the phenotype or make it so they’re incompatible with life
-Can cause cancers e.g Burkitt’s Lymphoma which is cancer of the white blood cells

Question 8

What is nondisjunction?

Answer 8

-Can affect the whole chromosomes
-Happens when the homologous pair fails to seperate during meiosis and there may be 2 or 0 gametes produced
-When an abnormal and normal gamete join it forms aneuploidy

Question 9

What happens in prophase 1?

Answer 9

Each chromosome is condensed into the form of two chromatids, homologous pairs join and crossing over happens

Question 10

What happens in metaphase 1?

Answer 10

The spindle fibres form and the pairs of chromosomes line up on the metaphase plate

Question 11

What happens in anaphase 1?

Answer 11

The centromeres don’t divide and one chromosome from each pair moves to the end of the cell, the chromosome number in the cell is now half of the original

Question 12

What happens in telophase 1?

Answer 12

The nuclear membrane re forms and the cells start to divide

Question 13

What happens in prophase 2?

Answer 13

New spindle fibres are made

Question 14

What happens in metaphase 2?

Answer 14

The chromosomes still made up with pairs of chromatids line up on the metaphase plate

Question 15

What happens in anaphase 2?

Answer 15

The centromeres now divide and the chromatids move to opposite ends of the cells

Question 16

What happens in telophase 2?

Answer 16

The nuclear envelope re forms and chromosomes return to interphase and cytokinesis starts

Question 17

How does gametogenesis start?

Answer 17

Primordial germ cells in the gonads divide, grow and divide again and then differentiate to form gametes

Question 18

What are the steps of sperm production?

Answer 18

-Diploid germ cell divides by mitosis and produces spermatagonia
-The spermatagonia grow until they become primary spermatocytes
-The spermatocytes divide by meosis to give two haploid secondary spermatocytes
-The secondary spermatocytes undergo meiosis again to form four haploid spermatids
-Spermatids then differentiate into spermatozoa which are capable of fertilisation

Question 19

What is the structure of sperm?

Answer 19

-An acrosome = digestive enzymes to digest outer layer of the ovum
-Haploid nucleus = highly condensed chromosomes
-Mitochondria = release energy for the tail to move
-Tail = contains microtubules which move the tail and keep the sperm in suspension

Question 20

What are the steps of egg production?

Answer 20

-The diploid germ cell divides by mitosis to form diploid oogonia
-The oogonium grows to form a diploid primary oocyte
-The primary oocyte undergoes meiosis to form a haploid secondary oocyte called ova and a first polar body
-Ovulation occurs
-The second division doesn’t occur until fertilisation
-The secondary oocyte divides by meiosis again to form an ovum and a second polar body
-The polar bodies then disintergrate

Question 21

What is the structure of the egg?

Answer 21

-Haploid nucleus
-Has cytoplasm with lots of food reserves
-A corona radiata which forms part of the protective barrier of the ovum
-A jelly coating for protection
-Zona pellucida protects the egg by hardening after fertilisation

Question 22

What is internal fertilisation?

Answer 22

Where the male gametes are transferred directly to the female

Question 23

What is external fertilisation?

Answer 23

Occurs outside the body and the gametes are released directly to the environment

Question 24

Where does fertilisation in the female happen?

Answer 24

The oviduct

Question 25

What are the stages for embryo development?

Answer 25

-Zygote is totipotent and can form all 216 cell types needed
-A type of mitosis (cleavage) happens where the cell divide rapidly with no interphase
-A blastocyst forms which is a mass of undifferentiated cells
-The cells become pluripotent and can differentiate into most cell types
-Outer layer of the blastocyst cells form the placenta
-Blastocyst breaks free and plants into the uterus