topic- 3.1/3.2

Question 1

define a gene

Answer 1

a heritable factor that consists of a length of DNA and influences a specific characteristics

Question 2

where are genes located?

Answer 2

a gene occupies a specific position on one type of chromosome- this is called a locus

Question 3

define an allele

Answer 3

alternative forms of the same gene

Question 4

alleles have the same —— and only one allele can occupy the —–

Answer 4

locus; the locus of the gene on a chromosome

Question 5

how do alleles differ from each other?

Answer 5

alleles differ from each other by one or a few bases only

Question 6

how many chromosomes do humans have?

Answer 6

46 chromosomes; 23 pairs of chromosomes, where half come from the mother and half come from the father

Question 7

how are new alleles formed?

Answer 7

new alleles are formed by gene mutation, or random changes to the base sequence of DNA

Question 8

what is a genome?

Answer 8

the whole of the genetic information of an organism

Question 9

what does the genome of humans consist of?

Answer 9

the 46 molecules that form the chromosomes in the nucleus plus the DNA molecule in the mitochondrion

Question 10

what does the genome of plants consist of?

Answer 10

the DNA molecules of chromosomes in the nucleus plus the DNA molecules in the mitochondrion and the chloroplast

Question 11

what does the genome of prokaryotes consist of?

Answer 11

the DNA in the circular chromosome, plus any plasmids that are present

Question 12

what was done during the human genome project?

Answer 12

the entire base sequence of human genes was sequenced

Question 13

what is the cause of sickle cell anaemia ?

Answer 13

• a mutation of the gene that codes for the beta-globin polypeptide in haemoglobin
• normal allele= Hb^A, 6th codon of gene is GAG
• sickle cell allele= Hb^s, 6th codon of gene is GTG
• mutation only inherited by offspring if it occurs in a cell of the very or testis that develops into egg or sperm
• when Hb^s allele is transcribed, the mRNA has GUG as its 6th codon instead of GAG, so when mRNA is translated the 6th AA in the polypeptide is valine instead of glutamic acid
• this causes haemoglobin molecules to stick together in tissues with low oxygen concentrations
• bundles of haemoglobin are rigid so distort the RBCs into a sickle shape

Question 14

give the consequences of sickle cell anaemia

Answer 14

• sickle cells become trapped in blood capillaries, blocking them and reducing blood flow
• both haemoglobin and plasma membrane damaged and life of a RBC shortened to as little as 4 days
• RBCs not replaced at a fast enough rate= anaemia develops

Question 15

describe prokaryotic genetic material

Answer 15

prokaryotes have one chromosome consisting of a circular DNA molecule, which is naked as it is not associated with proteins. There is a single copy of each gene

Question 16

what types of organisms contain plasmids?

Answer 16

some prokaryotes but NOT eukaryotes

Question 17

what are plasmids?

Answer 17

small extra DNA molecules that are commonly found in prokaryotes- they are usually small, circular and naked, containing few genes that may be useful to the cell but not those needed for its basic life processes

Question 18

give an example of a gene that may be in a plasmid

Answer 18

genes for antibiotic resistance

Question 19

what is the problem with plasmids?

Answer 19

they are not always replicated at the same time as the chromosomes of a prokaryotic cell or at the same rate.
- may not be passed on to both cells formed by cell division
- may be multiple copies of a plasmid in a cell

Question 20

what is different about plasmids?

Answer 20

copies of plasmids can be transferred from one cell to another, allowing spread through a population. they can also cross the species barrier.

Question 21

how did John Cairns measure the length of DNA?

Answer 21

by autoradiography

Question 22

describe eukaryotic genetic material

Answer 22

eukaryotic chromosomes are linear DNA molecules associated with histone proteins
different chromosomes carry different genes

Question 23

how many types of chromosome are there in humans?

Answer 23

23

Question 24

what are homologous chromosomes?

Answer 24

chromosomes that carry the same sequence of genes but not necessarily the same alleles of those genes

Question 25

compare the genome sizes of T2 Phage, E.Coli, Drosophila melanogaster, Homo sapiens and Paris Japonica

Answer 25

T2 Phage- virus- 0.18 m base pairs
E.coli- gut bacterium- 5 m bp
Drosophila melanogaster- fruit fly- 140 m bp
Homo sapiens- human- 3,000 m bp
Paris Japonica- woodland plant- 150,000 m bp

Question 26

describe haploid nuclei and give an example

Answer 26

have one chromosome of each pair (eg 23 in humans) human sex cells or gametes

Question 27

describe diploid nuclei

Answer 27

have pairs of homologous chromosomes
- have two copies of each gene, apart from genes on sex chromosomes

Question 28

give an advantage of having diploid nuclei

Answer 28

the effects of harmful recessive mutations can be avoided if a dominant allele is also present

Question 29

explain the importance of chromosome numbers

Answer 29

the number of chromosomes is a characteristic feature of members of a species

Question 30

compare the diploid chromosome numbers of Homo sapiens, pan troglodytes, Canis familiaris, oryza sativa, parascaris equorum

Answer 30

parascaris equorum- horse threadworm- 4
oryza sativa- rice- 24
Homo sapiens- 46
pan troglodytes- chimpanzee- 48
canis familiaris-dog- 78

Question 31

what is sex determined by?

Answer 31

sex chromosomes

Question 32

what are autosomes?

Answer 32

chromosomes that do not determine sex

Question 33

describe the 2 chromosomes that determine sex in humans

Answer 33

• the X chromosome is relatively large and has its centromere near the middle
• the Y chromosome is much smaller and has its centromere near the end

Question 34

why must all humans have the X chromosome?

Answer 34

it has many genes that are essential in both males and females

Question 35

what chromosome gene causes a foetus to develop as a male?

Answer 35

SRY or TDF- initiates the development of male features, including testes and testosterone production

Question 36

when is the sex of a human determined and why?

Answer 36

at fertilisation. this is because when sperm are formed, half contain the X chromosome and half the Y chromosome.

Question 37

what is a karyogram?

Answer 37

a karyogram shows the chromosomes of an organism in homologous pairs of decreasing length

Question 38

when can we get the clearest view of chromosomes?

Answer 38

during metaphase, in cells that are in mitosis

Question 39

how do we obtain a karyogram?

Answer 39

• dividing cells stained and placed on microscope slide
• burst
• chromosomes become spread
• micrograph taken