B13 Reproduction

Question 1

compare asexual and sexual reproduction

Answer 1

asexual
- cells divide by mitosis,
- offspring is genetically identical to (a clone of) parent: no variation,

sexual
- cells divide by meiosis to form gametes,
- gametes fuse to form a zygote,
- offspring inherits genetic information and characteristics from both parents: variation.

Question 2

in which part of the body are gametes produced in animals?
e.g. for humans?

Answer 2

in the sex organs
e.g. testes and ovaries.

Question 3

what are the animal
and the plant
gametes?

Answer 3

humans:
- female: egg cells,
- male: sperm

plants:
- female: egg cells,
- male: pollen.

Question 4

describe the process of meiosis of a cell

Answer 4

• cell copies its genetic information, so that there are now 4 (rather than 2) sets of each chromosome which each forms a pair of chromatids,
• cell divides twice in quick succession to form four gametes, each with one set of chromosomes.

Question 5

how many chromosomes does a human body cell have?
so how many chromosomes does a human gamete have?

Answer 5

46 (in 23 pairs)
gamete has 23

Question 6

advantages + disadvantages of asexual reproduction

Answer 6

asexual
advantages
- time + energy efficient: one parent so no need to find a mate or spread gametes,
- no variation in offspring, can produce large amount of clones which can be useful e.g. in agriculture.

disadvantage
- introduces no variation: if there is an unfavourable change in the environment, no organisms of the species can survive.

Question 7

advantages + disadvantages of sexual reproduction

Answer 7

asexual
advantage
- variation in offspring, higher chance of some surviving if there is a change in the environment, natural selection,

disadvantages
- time + energy inefficient: two parents so it takes energy and time to find a mate or spread gametes.

Question 8

describe how malarial parasites reproduce

Answer 8

• asexually in human liver and blood cells,
• when mosquito takes blood meal, the drop in temperature triggers sexual reproduction in parasites in the red blood cells,
• for 20 mins, sexual forms burst from the blood cells and meet to form zygotes with 2 sets of chromosomes,
• zygotes undergo meiosis to produce new asexual parasites to infect a new human host.

Question 9

what are chromosomes made of?

Answer 9

DNA

Question 10

what are genes?

Answer 10

sections of DNA which store genetic information

Question 11

each gene codes for…

Answer 11

each gene codes for a specific sequence of amino acids that forms a particular protein

Question 12

what is the genome of an organism?

Answer 12

the sum of all the organism’s genetic material

Question 13

why is studying the human genome important?

Answer 13

• helps us PREDICT and thus prevent the growth of DISEASES through lifestyle changes (since some genes are linked to certain diseases),
• helps us understand human evolution and history,
• helps us understand inherited disorders and how to overcome them.

Question 14

what is a nucleotide?
what do they make up?

Answer 14

• nucleotide: the combination of a phosphate and a sugar, which is attached to a base (A, T, C or G),
• repeating nucleotide units make up the DNA polymer.

Question 15

which bases always link up with each other in a DNA molecule?

Answer 15

A and T
G and C
(remember: AT Gemma Collins)

Question 16

why are the links of bases together important for the functioning of a DNA molecule?

Answer 16

• they hold the structure of the DNA double helix together,
• they help the information from genes on DNA be translated into proteins in the cell.

Question 17

describe the process of protein synthesis

Answer 17

• gene produces a template which reflects the order of bases given by DNA and is small enough to leave nucleus through pores in the nuclear membrane,
• template leaves nucleus and binds to surface of a ribosome,
• carrier molecules from cytoplasm attached to amino acids repeatedly bring and attach them to the template in the given order,
• until template is complete and a complete amino acid is formed,
• amino acids join together into proteins which are shaped for a specific function.

Question 18

why does changing the order of bases in the DNA affect the protein that is formed during protein synthesis?

Answer 18

• changed order of bases in DNA means a different template is formed,
• meaning it gives a different order of amino acids which will produce a different protein being produced by a gene.

Question 19

what is a mutation?

Answer 19

a change in an existing gene that results in a new gene

Question 20

what is an allele?

Answer 20

a form of a gene which codes for a particular protein and combine with each other to determine an offspring’s characteristics.

Question 21

how can a mutation affect the protein formed by a gene?

Answer 21

• most of the time only alter a protein’s appearance or function very slightly or not at all,
• sometimes a mutation can affect the shape of a protein which affects its function negatively (e.g. enzyme no longer binding to substrate) or positively (e.g. producing a more efficient enzyme).

Question 22

what is a homozygote
and a heterozygote?

Answer 22

homozygote: has two identical alleles for a characteristic,
heterozygote: has two different alleles for a characteristic.

Question 23

what is a genotype + example?

Answer 23

the alleles present in an individual regarding a particular characteristic,
e.g. Bb, bb.

Question 24

what is polydactyly?
what kind of allele is it caused by?

Answer 24

an inherited disorder which causes extra toes or fingers,
caused by a dominant allele.

Question 25

what is cystic fibrosis?
what kind of allele is it caused by?

Answer 25

an inherited cell membrane disorder that affects the function of certain organs,
caused by a recessive allele (so is a recessive phenotype).

Question 26

if an offspring inherits a disorder caused by a recessive allele
what can be said about the alleles of its parents?

Answer 26

its parents must both carry the recessive allele

Question 27

human female pair of chromosomes
human male pair of chromosomes

Answer 27

female: XX
male: XY

Question 28

what chromosome does
an egg cell
a sperm cell
contain?

Answer 28

• every egg cell contains an X chromosome,
• half of all sperm cells contain an X,
  half contain a Y.

Question 29

who is a ‘carrier’?

Answer 29

somebody who carries a recessive allele for a disorder but exhibits no symptoms of it.

Question 30

compare the two ways (non IVF) of screening an embryo or fetus

Answer 30

1) amniocentesis:
- carried out at 15-16 weeks of pregnancy,
- screen fetal cells from the fluid around the developing fetus,
- risk of miscarriage,

2) chorionic villus sampling:
-carried out at 10-12 weeks of pregnancy,
- screen fetal cells from tissue of developing
placenta,
- risk of miscarriage.

Question 31

5 arguments against embryo screening

Answer 31

• risk of miscarriage; could miscarry a healthy fetus,
• can give false negative/positive,
• ethical debate about terminating pregnancies,
• economic: screening is expensive,
• social: could give rise to ‘designer babies’.