Topic 4 - Genetics

Question 1

What is a gene?

Answer 1

Section of DNA that codes for making a polypeptide and functional RNA

Question 2

Define Locus

Answer 2

Location of a particular gene on a chromosome

Question 3

Define allele

Answer 3

Alternative form of a gene

Question 4

Describe how DNA is stored in eukaryotes

Answer 4

stored as chromosomes in the nucleus DNA is tightly wound around proteins called histones
Chromosomes are linear in shape

Question 5

Describe how DNA is stored in prokaryotes

Answer 5

DNA is shorter and circular
dna isn’t wound around histones

Question 6

Define homologous pairs

Answer 6

Pairs of matching chromosomes
the chromsomes are exactly the same size, have exactly the same genes but might have different alleles

Question 7

Define transcription

Answer 7

Where one gene on the DNA is copied into mRNA

Question 8

Define translation

Answer 8

Where the mRNA joins with a ribosome, and corresponding tRNA molecules brings the specific amino acid the codon codes for

Question 9

Give the first stage of transcription

Answer 9

DNA helix unwinds - DNA helices breaks hydrogen bonds - one chain of the DNA acts as a template

Question 10

Give the second stage of transcription

Answer 10

Free mRNA nucleotides in the nucleus align opposite exposed complementary DNA bases

Question 11

Give the third stage of transcription

Answer 11

RNA polymerase bonds together the RNA nucleotides to create a new RNA polymer chain. One entire gene is copied

Question 12

What happens to pre-mRNA after transcription?

Answer 12

Pre-mRNA has to be modified to become mRNA that is ready to leave the nucleus and take part in translation.
The introns need to be spliced out

Question 13

What are introns?

Answer 13

Introns don’t code amino acids

Question 14

Describe what happens to the introns after transcription

Answer 14

The introns are spliced out by a protein (splicesome). This just leaves the exons (coding regions)

Question 15

Describe the first stage of translation

Answer 15

Modified mRNA leaves the nucleus and attaches to a ribosome. Ribosome attaches at start codon. tRNA molecule with complementary anticodon to the start codon aligns opposite mRNA, held in place by the ribosome.

Question 16

Describe the second stage of translation

Answer 16

Ribosome will move along the mRNA molecule to allow other complementary tRNA to attach to the next codon on the mRNA.
The two amino acids that have been delivered by the tRNA molecule are joined by a peptide bond. This is catalysed by an enzyme and requires ATP.

Question 17

Describe the third stage of translation

Answer 17

This continues until the ribosome reaches the stop codon at the end of the mRNA molecule. The stop codon doesn’t code for an amino acid and therefore the ribosome detaches and translation ends.

Question 18

describe the DNA found in chloroplasts and mitochondria

Answer 18

similar to prokaryotic DNA, in that it’s short and circular

Question 19

what is a start codon

Answer 19

they are at the start of every gene and they code for an amino acid

Question 20

what is a stop codon

Answer 20

occur at the end of every gene that do not code for an amino acid

Question 21

what is meant by the genetic code being degenerate

Answer 21

64 combinations is more than is needed to code for 20 amino acids

each amino acid is coded by more than 1 triplet of bases

Question 22

what is meant by the genetic code being universal

Answer 22

the same triplet of bases codes for the same amino acids in all organisms

Question 23

what is meant by the genetic code being non-overlapping

Answer 23

each base in a gene is only part of one triplet of bases that codes for one amino acid

Question 24

describe meiosis

Answer 24

produces 4 daughter cells that are genetically different
4 haploid daughter cells
2 nuclear divisions
haploid cell
introduces genteic variation

Question 25

describe the process of crossing over (4 marks)

Answer 25

in meiosis I, parts of the chromatid can become twisted around each other this puts tension on the chromatids, causing parts of the chromatid to break the broken parts of the chromatid recombine with the another chromatid results in new combinations of alleles

Question 26

describe independent segregation (3 marks)

Answer 26

in meiosis I, homologous pairs of chromosomes line up opposite each other at the equator of the cell it's random which side of the equator the paternal and maternal chromosomes from each homologous pair lie these pairs are separated, so one of each homologous pair ends up in the daughter cell

Question 27

give the definition of genetic diversity

Answer 27

the number of different alleles of genes in a population

Question 28

when can natural selection occur

Answer 28

only occur when there is genetic diversity within the population

Question 29

define evolution

Answer 29

the change in allele frequency over many generations in a population

Question 30

what does natural selection result in

Answer 30

species becoming better adapted to their environment, adaptations may be anatomical, physiological or behavioural

Question 31

describe the process of natural selection (4 marks)

Answer 31

new alleles for a gene are created by random mutations if the new alleles increases the chances of the individual to survive in that environment, then they are more likely to survive and reproduce this reproduction passes on the advantageous allele to the next generation over many generations the new allele increases in frequency in the population

Question 32

why is courtship essential

Answer 32

essential for successful mating and species recognition

Question 33

define species

Answer 33

a group of similar organism that can breed to make fertile offspring

Question 34

what are courtship rituals

Answer 34

sequence of actions which is unique to a species most courtship rituals are performed by males they can include a sequence of dance moves, sounds, colourful feathers

Question 35

why is courtship important to ensure successful reproduction?

Answer 35

enables them to recognise own species and opposite sex synchronises mating behaviour - indicates sexually mature and in season (releasing egg)

Question 36

why is courtship important to ensure the survival of the offspring?

Answer 36

form a pair bond choose a strong and healthy mate

Question 37

give the two types of selection

Answer 37

directional selection stabilising selection

Question 38

describe directional selection and give an example

Answer 38

one of the extremes has the selective advantage occurs when there is a change in the environment the modal trait changes e.g. antibiotic resistance in bacteria

Question 39

describe stabilising selection and give an example

Answer 39

the modal trait has the selective advantage occurs when there is no change in the environment modal trait remains the same standard deviation decreases, as individuals with the extreme trait decrease e.g. human birth weights

Question 40

when can chromosome mutations arise

Answer 40

can arise spontaneously by chromosome non-disjunction during meiosis

Question 41

define non-disjunction

Answer 41

when the chromosomes or chromatids do not split equally during anaphase

Question 42

describe polyploidy

Answer 42

changes in whole sets of chromosomes occur when organisms have three or more sets of chromosomes rather than the usual 2

Question 43

describe aneuploidy (3 marks)

Answer 43

changes in the number of individual chromosomes sometimes individual homologous pairs of chromosomes fail to separate during meiosis this is non-disjunction and usually results in a gamete having one more or one fewer chromosome

Question 44

describe binomial system

Answer 44

first name genus second name species same genus shows a close relationship

Question 45

why do different species look similar?

Answer 45

live in a similar environment have similar selection pressures similar alleles will have the selective advantage

Question 46

define hierarchy

Answer 46

smaller groups arranged within larger groups no overlap between groups

Question 47

give more modern and accurate classification methods

Answer 47

DNA sequence mRNA sequence amino acids sequence

Question 48

define phylogenetic classification

Answer 48

arranges species into groups according to their evolutionary origins and relationships