Genes...

Front 1

Explain how meiosis can result in genetic variety in gametes. (8)

Back 1

meiosis provides opportunities for new combination of genes;
crossing over in prophase I;
due to chiasmata formation;
segments of (non-sister) chromatids are exchanged between homologous chromosomes;
chromosome mutations could occur;
leading to the formation of new combination of alleles (on chromosomes of gametes) /
the formation of new linkage groups;
the random orientation of homologous chromosomes/bivalents in metaphase I;
gives independent assortment of (maternal and paternal) chromosomes in anaphase;
as homologous chromosomes migrate to opposite poles randomly;
223 possible gametes;
non-disjunction could occur leading to diploid gametes/trisomy;
random orientation in metaphase II;
gives the independent assortment of chromatids in anaphase II

Front 2

Outline how the chi-square test can be used in analyzing monohybrid crosses. (6)

Back 2

Chi squared test is used to determine whether a difference between expected and observed results is due to chance;
tests the significance of data;
data must consist of discrete / discontinuous variables;
between observed (O) and expected result (E);
the hypothesis to be tested (the null hypothesis) is that there are no significant differences between them;
(Chi-squared) = Σ x (O E)^2 / E ;
example given e.g. values for O and E in cross between tall and dwarf;
establish degree of freedom;
statistical tables used to identify probability;
critical value of 5 %;

Front 3

Describe the inheritance of human skin colour. (5)

Back 3

characteristic caused by the combined effect of more than one gene;
example of polygenic inheritance;
linked to human melanin production;
skin colour shows continuous variation;
phenotypes do not follow simple Mendelian ratios of dominance and recessiveness;
the more loci/alleles concerned, the greater the number of phenotypic classes;
giving different shades of skin colour;
the environment also affects gene expression of skin colour / sunlight/UV light
stimulate melanin production;
the more recessive alleles there are, the lighter the skin colour;

Front 4

Describe the relationship between genes, polypeptides and enzymes. (4)

Back 4

a. gene is a sequence of DNA bases;
b. DNA/gene codes for a specific sequence of amino acids/polypeptide;
c. enzymes are proteins/composed of polypetides;
d. sequence of amino acids determines tertiary structure/folding/shape of active site;
e. change in the gene/mutation will affect the active site/function of an enzyme;
f. enzymes are involved in replication/transcription of genes;
g. enzymes are involved in synthesis of polypeptides;

Front 5

Distinguish between the sense and antisense strands of DNA during transcription. (1)

Back 5

only the antisense strand is transcribed / the antisense strand is transcribed to mRNA and the sense strand is not transcribed/has the same base sequence as mRNA (with uracil instead of thymine)

Front 6

Compare the DNA found in prokaryotic cells (PC) and eukaryotic cells (EC). (2)

Back 6

PC = circular EC = linear

PC=in cytoplasm EC=enclosed in the nuclear membrane

PC=naked EC=associated with proteins/histones

PC=plasmids EC=no plasmids

both prokaryotic and eukaryotic DNA consist of a double helix of (deoxy)nucleotides / phosphate, deoxyribose and base/ATC and G;

Front 7

Outline the various stages of the cell cycle. (4)

Back 7

G1 the cell grows/duplication of organelles;
S is synthesis stage when DNA is synthesized/replicated;
G2 the chromosomes begin condensing/preparation for cell division;
G1 , S and G2 make up interphase;
during mitosis nuclear division occurs/all four stages listed;
during cytokinesis cytoplasm/cell divides/daughter cells formed;

Front 8

Explain how an error in meiosis can lead to Down syndrome. (8)

Back 8

non-disjunction;
chromosomes/chromatids do not separate / go to same pole;
non-separation of (homologous) chromosomes during anaphase I;
due to incorrect spindle attachment;
non-separation of chromatids during anaphase II;
due to centromeres not dividing;
occurs during gamete/sperm/egg formation;
less common in sperm than egg formation / function of parents' age;
Down syndrome due to extra chromosome 21;
sperm/egg/gamete receives two chromosomes of same type;
zygote/offspring with three chromosomes of same type / trisomy / total 47 chromosomes;

Front 9

List two examples of how human life depends on mitosis.

Back 9

growth/production of (extra) body cells;
first stage of spermato/oo/gametogenesis / forming oogonia/spermatogonia;
embryo development;
wound healing / (tissue) repair / hair growth / replacement of
skin cells;
clonal selection / division of lymphocytes (for antibody production);

Front 10

Describe the importance of stem cells in differentiation. (3)

Back 10

stem cells are undifferentiated cells;
embryo cells are stem cells;
stem cells can differentiate in many/all ways / are pluripotent/totipotent;
differentiation involves expressing some genes but not others;
stem cells can be used to repair/replace tissues/heal wounds;

Front 11

Outline the bonding between DNA nucleotides. (2)

Back 11

hydrogen bonds between nucleotides on opposite strands/complementary bases/adenine and thymine and cytosine and guanine;
covalent bonds between nucleotides within strands/between sugar/deoxyribose
and phosphate;

Front 12

Explain why carriers of sex-linked (X-linked) genes must be heterozygous. (2)

Back 12

carrier has (one copy of) a recessive allele;
must also have a dominant allele to prevent having the condition/disease; [2]
or
cannot be homozygous dominant or they would not carry the recessive allele;
cannot be homozygous recessive or they would have the condition/disease;

Front 13

State two general types of enzymes used in gene transfer.

Back 13

restriction enzymes/endonucleases
ligases
reverse transcriptase

Front 14

Outline the structure of a ribosome. (4)

Back 14

small subunit and large subunit;
mRNA binding site on small subunit;
three tRNA binding sites / A, P and E tRNA binding sites;
protein and RNA composition (in both subunits);

Front 15

Outline the formation of chiasmata during crossing over. (5)

Back 15

crossing over/chiasmata formed during prophase I of meiosis;
pairing of homologous chromosomes/synapsis;
chromatids break (at same point); (do not accept chromatids overlap),
non-sister chromatids join up/swap/exchange alleles/parts;
X-shaped structure formed / chiasmata are X-shaped structures;
chiasma formed at position where crossing over occurred;
chiasmata become visible when homologous chromosomes unpair;
chiasma holds homologous chromosomes together (until anaphase);

Front 16

Outline the structure of nucleosomes. (2)

Back 16

(eight) histone (proteins);
DNA wrapped around histones/nucleosome;
further histone holding these together;

Front 17

Define the term polygenic inheritance.

Back 17

character/affected/influenced/defined/determined/ controlled by two or more genes

Front 18

Explain, using a named example, how polygenic inheritance gives rise to continuous variation. (4)

Back 18

human skin colour can vary from pale to very dark / amount of melanin varies;
skin colour/melanin controlled by (alleles from) at least three/several genes;
no alleles are dominant / alleles are co-dominant / incomplete dominance;
many different possible combinations of alleles;
skin colour controlled by cumulative effect/combination of genes/alleles;

Front 19

Describe the inheritance of colour blindness in humans. (3)

Back 19

sex linked condition;
carried on an X chromosome / absent from Y chromosome;
if present in male causes colour blindness;
(allele is) recessive so heterozygous females are not colour blind;
homozygous females are colour blind;

Front 20

Explain prokaryotic DNA replication. (8)

Back 20

DNA replication is semi-conservative / each strand of DNA acts as template;
(DNA) helicase separates two strands/forms a replication fork;
new strand built / nucleotides added in a 5' to 3' direction;
(deoxy)nucleoside triphosphates hydrolysed to provide energy for nucleotide formation/base pairing;
on one strand DNA polymerase III builds continuous strand;
on other strand short chains of DNA/Okazaki fragments are formed;
each short chain starts with RNA primer;
added by RNA primase;
then remainder of chain of DNA built by DNA polymerase III;
DNA polymerase I removes RNA primer and replaces it by DNA;
DNA ligase joins DNA fragments together forming complete strand;
replication only occurs at a single replication fork;

Front 21

Outline a basic technique for gene transfer involving plasmids. (6)

Back 21

gene transfer takes a gene from one species/organism and inserts it into another;
plasmid – small loops of DNA in bacteria;
remove plasmid from bacteria;
restriction enzyme/endonuclease cuts/cleaves (at target sequence) in plasmid;
sticky ends left/made/added at ends of cut plasmid;
isolate mRNA of specific gene;
DNA copies made with reverse transcriptase / cut donor DNA with the same
restriction enzyme;
DNA ligase joins the DNA (to sticky ends) of open plasmid;
(results in) recombinant plasmid;
uptake/insert plasmid to (host) cell;
clone cells;

Front 22

Explain how sexual reproduction promotes variation in a species. (8)

Back 22

meiosis results in four haploid cells/gametes;
random assortment of chromosomes;
in metaphase I;
gives rise to variety of haploid gametes;
2n possible gametes where n is the haploid number;
crossover may occur between homologous chromosomes;
in prophase I;
causes new combinations of genetic material/alleles;
non-disjunction causes changes in chromosome numbers;
infinite variety in gametes;
random process of fertilization;
random process of mating;
new combinations even with same parents;
mutation can occur in prophase I e.g. deletion / inversion / translocation;

Front 23

Compare the structure and composition of DNA with RNA. (4)

Back 23

both are polymers of nucleotides / both nucleic acids;
sugar is deoxyribose in DNA and ribose in RNA;
DNA is double stranded and RNA is single stranded;
DNA has a (double) helix;
DNA has thymine while RNA has uracil;
both contain four nitrogenous bases / A, G, C, T for DNA and A, G, C, U for RNA;

Front 24

Describe the characteristics of stem cells that make them potentially useful in medicine (5)

Back 24

a. (stem cells) have/retain the capacity to divide;
b. can be used to produce cell cultures/large number of identical cells;
c. can be used to repair/replace damaged/lost cells/tissue;
d. (stem cells) are undifferentiated / have not yet differentiated/specialized;
e. can differentiate/specialize in different ways / are pluripotent/totipotent;
f. can be used to form a variety of different tissues / form organs;
g. used in medical research;
h. used in treatment of (named) disease;

Front 25

Explain the use of karyotyping in human genetics. (8)

Back 25

Definition and construction of karyotypes:
a. karyotype is the number and type / image of chromosomes in a cell;
b. cells collected from chorionic villus / by amniocentesis;
c. requires cells in metaphase / stimulate cells to divide and reach metaphase;
d. burst cells and spread chromosomes / photo taken of chromosomes;
e. chromosomes are arranged in pairs;
f. according to size/structure/position of centromere/banding pattern;

Uses for karyotypes:
g. karyotypes used to identify sex/gender;
h. male is XY and female XX;
i. used to identify chromosome mutations/abnormal numbers/non-disjunction;
j. Down syndrome due to extra chromosome 21 / other trisomy/aneuploidy example;
k. used for pre-natal diagnosis of chromosome abnormalities;
l. may lead to a decision to abort the fetus;
m. prepare for consequences of abnormality in offspring;

Front 26

Explain the processes that result in genetic variation in the sperm produced by an adult male. (5)

Back 26

meiosis;
independent assortment/random orientation of bivalents/pairs of
chromosomes/homologous chromosomes;
in metaphase I;
223/2n possible combinations (where n is the haploid number of chromosomes);
crossing over / recombination of linked genes;
during prophase I;
can occur anywhere along a chromosome;
random orientation of chromatids (in metaphase II);
(gene) mutations may occur;

Front 27

features of a homologous chromosome

Back 27

paired/formed a bivalent / tetrad / there is
crossing over between the chromosomes / they have the same genes (in the same sequence) / they are the same size and shape

Front 28

what are recombinants of linked genes

Back 28

combinations of genes not found in parents

Front 29

Translation occurs in living cells. Explain how translation is carried out, from the initiation stage onwards. (9)

Back 29

translation involves initiation, elongation/translocation and termination;
mRNA binds to the small sub-unit of the ribosome;
ribosome slides along mRNA to the start codon;
anticodon of tRNA pairs with codon on mRNA:
complementary base pairing (between codon and anticodon);
(anticodon of) tRNA with methionine pairs with start codon / AUG is the start codon;
second tRNA pairs with next codon;
peptide bond forms between amino acids;
ribosome moves along the mRNA by one codon;
movement in 5 to 3 direction;
tRNA that has lost its amino acid detaches;
another tRNA pairs with the next codon/moves into A site;
tRNA activating enzymes;
link amino acids to specific tRNA;
stop codon (eventually) reached;

Front 30

Explain the methods and aims of DNA profiling. (8)

Back 30

DNA sample obtained;
from hair/blood/semen/human tissue;
DNA amplified / quantities of DNA increased by PCR/polymerase chain reaction;
satellite DNA/highly repetitive sequences are used/amplified;
DNA cut into fragments;
using restriction enzymes/restriction endonucleases;
gel electrophoresis is used to separate DNA fragments;
using electric field / fragments separated by size;
number of repeats varies between individuals / pattern of bands is unique to the
individual/unlikely to be shared;

forensic use / crime scene investigation;
example of forensic use e.g. DNA obtained from the crime scene/victim compared
to DNA of suspect / other example of forensic use;
paternity testing use e.g. DNA obtained from parents in paternity cases;
biological father if one half of all bands in the child are found in the father;
genetic screening;
presence of particular bands correlates with probability of certain phenotype /
allele;

Front 31

Distinguish between RNA and DNA. (3)

Back 31

DNA is double-stranded while RNA is single-stranded;
DNA contains deoxyribose while RNA contains ribose;
the base thymine found in DNA is replaced by uracil in RNA;
one form of DNA (double helix) but several forms of RNA (tRNA, mRNA and rRNA);

Front 32

Outline the consequences of a base substitution mutation. (2)

Back 32

changes triplet code/codon;
different amino acid (may be) coded for/inserted;
(may) change protein/polypeptide/primary structure/sequence of amino acids / may code for a different protein;
may cause sickle cell anemia/other correctly named disease / form stop codon;

Front 33

Mendel crossed tall, round-seeded plants with short, wrinkled-seeded plants.
All F1 produced were tall, round-seeded plants. When F1 plants were crossed
with other F1 plants, the F2 generation produced many more than 1/16 short,
wrinkled-seeded plants. Deduce, with reasons, the inheritance of these genes.

Back 33

dihybrid cross would give 9:3: 3:1 ratio in this F2 if genes not linked;
(give credit for punnett grid showing this)
therefore the genes are linked;
more homozygous recessive offspring / 3:1 ratio expected if linked;

Front 34

Explain why the process used during protein synthesis in cells is called translation. (2)

Back 34

codon/triplet of bases to amino acid;
nucleic acid / base sequence / (m)RNA to polypeptide / protein / amino acid sequence;
genetic code has to be translated;

Front 35

Explain briefly how termination of translation occurs. (2)

Back 35

stop/terminator / nonsense codon (is reached);
polypeptide is released;
mRNA detaches from ribosome;
subunits of ribosome separate;