TOPIC 4 - CH 9

Question 1

What are homologous chromsomes?

Answer 1

2 chromosomes that have the same genes, but not necessarily the same alleles

Question 2

What is the importance of meiosis?

Answer 2

to maintain a constant number of chromosomes in the adults of a species

in animals meiosis occus in the formation of gametes,
for ferns meiosis occurs in the formation of spores

Question 3

What is independent segregation of homologous chromosomes?

Answer 3

• in meiosis I, homologous pairs of the chromosomes line up opposite of each other at the equator of the cell
• it is random which sde of the equator the paternal and the marternal chromosomes lie
• these pairs are separated one of each homolgous pair ends up in the daughter cell

Question 4

What are the 2 ways meiosis brings about genetic variation?

Answer 4

1. independent segregation of homologous chromosomes
2. crossing over between homologous chromosomes

Question 5

What is crossing over?

Answer 5

• During meiosis I, homolgous paris of chromosomes associate to form a bivalent
• non-sister chromatids netangl and chiasmata form at point where they touch
• at each chiasma, equal lngths of non-sister chromatids are exchanged
• this results in new combinations of alleles on the chromatids

Question 6

Differences between meiosis and mitosis

Answer 6

• Meiosis involves 2 nuclear divisions, only 1 in mitosis
• Haploid cell produced in meiosis, diploid cell in mitosis
• Meiosis introduces genetic variation, mitosis creates genetically identical cells
• Meiosis produces 4 daughter cells (2n)each with half the number of chromosomes (n) as the parent cell, Mitosis produces 2 daughter cells with the same number of chromosomes as the parent cell and as each other

Question 7

What is a gene mutation?

Answer 7

a change in the base sequence of DNA
this occurs randomly during DNA replication and can be inherited during the formation of gametes

Question 8

When are gene muations more likely to occur?

Answer 8

If you are exposed to mutagenic agents which can interfere with DNA replication

Question 9

What are some examples of mutagenic agents?

Answer 9

• High energy radiation - UV light
• Ionising radiation - Gamma and X rays
• Chemicals - carcinogens eg cigarette smoke

Question 10

What are the 2 ways bases can be impacted by a gene mutation?

Answer 10

A base could either be deleted or substituted

Question 11

Why is substitution less harmful than deletion?

Answer 11

substitution may mean that the new codon still codes for the same amino acid because the genetic code is degenerate

deletion results in a frameshift, removal of one base changes all the subsequent codons so multiple amino acids may be incorrectly coded for

Question 12

What are chromosome mutations?

Answer 12

changes in the structure or number of whole chromosomes
This is can arise spontaneously and is named non-disjunction

Question 13

What does changes in whole sets of chromosomes mean?

Answer 13

when organisms have 3 or more sets of chromosomes rather than 2
this is called polyploidy
this is also non-disjunction

Question 14

What is non-disjunction?

Answer 14

1. changes in the number of individual chromosomes
   - when homologus chromosomes in meiosis I fail to separate
   or
   - when sister chromatids in meiosis II fail to separate
2. changes in whole sets of chromosomes
   when organisms have 3 or more sets of chromosomes rather than 2
   this is called polyploidy

Question 15

What is genetic diversity?

Answer 15

the total number of different alleles in a population

Question 16

What is a population?

Answer 16

a group of individuals of the same species that live in the same place
and can interbreed

Question 17

What is a species?

Answer 17

Consists of 1 or more populations

Question 18

What deteremines how diverse a population is?

Answer 18

greater number of different alleles = more diverse

Question 19

Why is genetic diversity beneficial?

Answer 19

more likely that some individuals in a population will survive an environmental change
because of wider range of alleles = wider range of characteristics
this enables natural selection to occur

Question 20

When can natural selection occur?

Answer 20

If there is genetic diversity within a population

Question 21

What is a gene pool?

Answer 21

All th genes and alleles in a population at a particular time

Question 22

What is the allele frequency?

Answer 22

The proportion of organisms within the population carrying a particular gene

Question 23

How does natural selection occur in terms of genetic diversity?

Answer 23

only certain individuals are reproductively successful and so pass on their genes
so not all alleles are equally likely to be passed on
this results in species becoming better adapted to their environment

Question 24

What is evolution?

Answer 24

The change in allele frequency over many generations in a population
Natural selection leads to this

Question 25

What are the 3 differents ways species can adapt?

Answer 25

- Anatomical - Phsysiological - Behavioural

Question 26

Describe the process of natural selection

Answer 26

1) Random mutations of alleles within the gene pool this introduces genetic variation 2) Mutation may give rise to new alleles that provide an orgnaism with an advantage to survive in its environment 3) The individual with the allele may be more likely to survive and reproduce and pass on the advantageous allele to their offspring 4) Over many generations, there will be an increase in the frequency of this allele in the population

Question 27

What is selection?

Answer 27

the process by which organisms that are better adapted to their environement tend to survive and breed while those that are less adapted tend not to

Question 28

What are the 2 types of selection?

Answer 28

- DIrectional selection - Stabilising selection

Question 29

What is directional selection?

Answer 29

One of the extremes has the seelective advantage -> occurs when there is a change in the environemnt the modal trait changes to one side one extreme is selected for, other extreme is selected against

Question 30

What is an example of directional selection?

Answer 30

Antiobiotic resistance in bacteria

Question 31

What is the effect of the environement on polygenes?

Answer 31

Polygenes influence characteristics This effect produces individuals that vary about the mean

Question 32

What is stabilising selection?

Answer 32

the modal trait remains the same as it has the selective advantage -> occurs when there is no change in the environment standard deviation decreases as phenotypes at extreames decrease mean is selected for, extremes selected against

Question 33

What may happen if there is an extreme change to the environment?

Answer 33

There may be no phenotype suited to the new conditions so the population will die out

Question 52

Why are animals with an odd number of chromosomes sterile (eg mules)?

Answer 52

Precise chromosome pairings are impopssible this prevents meiosis and gamete production

Question 52

Explain how a differencee in its DNA might lead to an organism having a different appearance and hence the species showing greater genetic diversity

Answer 52

Different DNA -> different codees for amino acids -> different amino acids -> different protein shape -> different protein function -> change in feature determined by protein -> altered appearance -> greater genetic diversity

Question 52

Explain why errors in DNA replication can be more damaging than errors in transcription

Answer 52

These errors may be inherited theerefore have a permanent affect on the whole organism Transcription errors affect only specific cells and are temporary and not inherited

Question 52

What is the distribution curve of directional selection like?

Answer 52

remains the same shape but mean shifts to the left or right

Question 52

What is the distribution curve of stabilising selection like?

Answer 52

becomes narrower and higher but the mean does not change