Cellular Control

Question 1

What are the stages in meiosis?

Answer 1

Prophase 1, Metaphase 1, Anaphase 1, Telophase 1, Prophase 2, Metaphase 2, Anaphase 2, Telophase 2

Question 2

Define meiosis

Answer 2

A reduction division. The resulting daughter cells have half the original number of chromosomes. The are haploid and can be used for sexual reproduction

Question 3

What happens in prophase 1?

Answer 3

The chromatin condenses and chromosomes shorten and thicken. Homologous pairs of chromosomes come together a bivalent. The non sister chromatids wrap around each other and attach at points called chiasmata. Crossing over occurs. The nucleolus disappears and nuclear envelope disintegrates. A spindle forms made of protein microtubules.

Question 4

What happens in metaphase 1?

Answer 4

Bivalents line up across the equator of the spindle, attached to spindle fibres at the centromere. Bivalents are orientated at random.

Question 5

What happens in anaphase 1?

Answer 5

Homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles; centromeres don’t divide. Chiasmata separate and lengths of chromatid that have been crossed over remain with the chromatid to which they have become newly attached.

Question 6

What happens in telophase 1?

Answer 6

In most animal cells, 2 new nuclear envelopes form, one around each set of chromosomes; cell divides by cytokinesis. A brief interphase occurs and chromosomes uncoil. In most plant cells, the cell goes straight from anaphase 1 to meiosis 2

Question 7

What happens in prophase 2?

Answer 7

If a nuclear envelope has reformed, it breaks down. The nucleolus disappears, chromosomes condense, spindles form

Question 8

What happens in metaphase 2?

Answer 8

The chromosomes arrange themselves on the equator of the spindle. They are attached to the spindle fibres at the centromeres. The chromatids of each chromosome are randomly orientated.

Question 9

What happens in anaphase 2?

Answer 9

The centromeres divide and chromatids are pulled to opposite poles by the spindle fibres.

Question 10

What happens in telophase 2?

Answer 10

Nuclear envelopes reform around haploid daughter nuclei. In animals, the 2 cells how divide to give 4 haploid cells. In plants, a tetrad of 4 haploid cells is formed.

Question 11

What do genes code for?

Answer 11

Polypeptides, including enzymes

Question 12

What is the genetic code?

Answer 12

The sequence of nucleotide bases on a gene provides a code with instructions for the makings of a protein. It has several characteristics: triplet code (64 different combinations); degenerate code; code for amino acid/stop codon; widespread (almost universal).

Question 13

Describe the process of transcription

Answer 13

Single stranded mRNA is constructed from the template strand of DNA. The DNA unwinds and unzips, breaking hydrogen bonds; activated RNA nucleotides (with 2 extra phosphoryl groups) bases bind to their complementary bases, catalysed by RNA polymerase; phosphoryll groups are released, releasing energy and sealing sugar/phosphate backbone; mRNA leaves nucleus via nuclear envelope.

Question 14

Describe the process of translation

Answer 14

mRNA binds to ribosome, 2 codons attached to the small subunit. The first is always methionine. tRNA brings the corresponding amino acid to the ribosome, and the codon and anticodon form hydrogen bonds. A second tRNA molecule brings the second amino acid. A peptide bond forms between the 2 amino acids. Ribosome moves along to the next codon, attaching a further amino acid. The polypeptide chain grows until a stop codon is reached.

Question 15

What are mutations?

Answer 15

Changes in the sequence of nucleotides in DNA molecules. May be deletion, addition or substitution

Question 16

What does cyclic AMP do to proteins?

Answer 16

Activates proteins by altering their 3-D structure.

Question 17

How can mutations affect proteins?

Answer 17

Could be silent: same amino acid is inserted, so no effect on function of protein
Could be harmful: different amino acid inserted/frame shift occurs, altering protein structure significantly, preventing it from functioning properly
Could be beneficial: amino acid inserted causes protein to function better, giving a genetic advantage.

Question 18

Describe how the lac operon works

Answer 18

Works to ensure the 2 enzymes needed to metabolise lactose, beta galactosidase and lactose permease are only produced when required.
Structural genes: code for beta-galactosidase and lactose permease
Operator region next to structural genes
Promoter region next to operator, where RNA polymerase binds
Regulator gene makes a protein. If no lactose present, it binds over the promoter and operator, preventing RNA polymerase from binding and thus the structural proteins aren’t produced
If lactose is present, it binds to the allosteric site, causing a shape change in the repressor protein, so it no longer binds to the operator.
RNA polymerase can now bind to the DNA, so mRNA is produced and the structural proteins are made.

Question 19

What are homeobox genes?

Answer 19

Genes that control the body plan of organisms. They control polarity: anterior (head)/posterior (tail). The homeobox genes cause segments to form and control the development of each individual segment. They are similar in plants, animals and fungi. They tend to be transcription factors that bind to DNA, allowing some proteins necessary to that segment to be produced. They are activated in the order that they are found in on the genome.

Question 20

What are Hox clusters?

Answer 20

Clusters of homeobox genes. Vertebrates have more Hox clusters than invertebrates, probably due to a duplication.

Question 21

What is apoptosis?

Answer 21

Programmed cell death. Occurs after a cell has undergone a certain number of divisions - 50 (the Hayflick constant)

Question 22

What is the sequence of events in apoptosis?

Answer 22

Enzymes break down the cell cytoskeleton
Cytoplasm becomes dense with tightly packed organelles
Cell surface membrane changes and small blebs form
Chromatin condenses and nuclear envelope breaks. DNA forms fragments
Cell breaks up into vesicles that are taken up by phagocytosis. Cellular debris is disposed of.

Question 23

What is the function of apoptosis?

Answer 23

A mechanism to change body plans. Causes digits to separate from each other. Not enough apoptosis causes tumours, too much causes cell loss and degeneration.

Question 24

Define allele

Answer 24

A version of a gene. A gene is a length of DNA that codes for one or more polypeptides. An allele of the gene has a difference in the DNA base sequence that is expressed as it translates into a slightly different polypeptide

Question 25

Define locus

Answer 25

The position of a gene on a chromosome

Question 26

Define phenotype

Answer 26

Refers to the characteristics that are expressed in the organism, meaning those features that can be observed. Determined by the genotype.

Question 27

Define genotype

Answer 27

The genetic make up of an organism, in terms of the alleles it possesses, usually in the context of a particular characteristic. The genotype determines the phenotype.

Question 28

Define dominant

Answer 28

An allele is dominant if it is always expressed in the phenotype, even if a different allele for the same gen is present in the genotype. they characteristic in question and its inheritance pattern are also described as dominant.

Question 29

Define codominant

Answer 29

2 alleles of the same gene are said to be codominant if they are both expressed in the phenotype of a heterozygote.

Question 30

Define recessive

Answer 30

An allele is recessive if it is only expressed in the phenotype if another identical allele/absence of a dominant allele is present.

Question 31

Define crossing-over

Answer 31

When lengths of DNA are swapped from one chromatid to another. Occurs during prophase 1 of meiosis. Homologous chromosome pairs form a bivalent, joining at chiasmata, with non-sister chromatids wrapping around each other. Chromosomes may break off and switch with their non-sister pair in crossing over, producing a new combination of alleles

Question 32

Define linkage

Answer 32

When 2 or more genes are located on the same chromosomes. They are normally inherited together, unless chiasmata have been formed between them and crossing over occuring.

Question 33

How do meiosis and fertilisation lead to genetic variation?

Answer 33

• Crossing over in prophase 1 shuffles alleles
• Genetic reassortment due to the random distribution and subsequent segregation of maternal and paternal chromosomes in homologous pairs, during meiosis 1
• Genetic reassortment due to the random distribution and segregation of the sister chromatids at meiosis 2
• Random mutation
• Random combination of 2 sets of chromosomes, one from each of 2 genetically unrelated individuals (fertilisation)

Question 34

What is epistasis?

Answer 34

The interaction of different gene loci so that one gene locus masks or suppresses the expression of another gene locus.

Question 35

Describe recessive epistasis

Answer 35

A recessive allele being present at one locus masks the expression of an allele at another locus. Often has a 9:3:4 ratio

Question 36

Describe dominant epistasis

Answer 36

A dominant allele being present at one locus masks the expression of an allele at another locus. Often has a 12:3:! ratio, or a 13:3

Question 37

What is the chi-squared test?

Answer 37

A statistical test that determines how likely there is a significant difference from the null hypothesis. First calculate expected values based on null hypothesis (null hypothesis is that it occurred due to normal meiosis, e.g. 9:3:3:1 ratio). Then use formula to calculate chi-square value and compare to data base at correct number of degrees of freedom (number of categories - 1)

Question 38

What are the differences between discontinuous and continuous variation?

Answer 38

Discontinuous: qualitative difference in phenotypes; clear categories; no intermediates; monogenic/epistatic; dominant/codominant/recessive patterns of inheritance
Continuous: quantitative difference in phenotypes; wide range of variation; no distinct categories; polygenic, additive component of each gene; affected by environment.

Question 39

What contributes to phenotypic variation?

Answer 39

Genetics AND environment.

Question 40

Why is variation essential?

Answer 40

Allows natural selection to occur: those with variations better suited to their environment will survive and reproduce, those without beneficial variations won’t.

Question 41

What are the assumptions that allow the Hardy-Weinberg principle to work?

Answer 41

Large population (no sampling errors)
Mating is random
No selective advantage for any genotype
No mutaion, migration or genetic drift

Question 42

What are the 2 equations for the Hardy-Weinberg principle?

Answer 42

p + q = 1
p^2 + 2pq + q^2 = 1
-p is dominant allele frequency
-q is recessive allele frequency
-q^2 is frequency of homozygous recessive
-2pq is frequency of heterozygous
-p^2 is frequency of homozygous dominant.
Find q^2 first from question, then find q, then p, then p^2, then 2pq

Question 43

How does the environment contribute to stabilising populations?

Answer 43

Populations don’t always increase in size, they sit at their carrying capacity. This is because environmental factors limit population growth, such as space, food, light, minerals, water availability. These factors causes the population to fluctuate around a mean level.
If the environment is ‘stable’, selection pressures will select for the modal characteristic. This is stabilising selection.

Question 44

How does the environment contribute to evolutionary force?

Answer 44

Not all organisms in a population survive to adulthood due to limitations in environmental factors. Those that survive to adulthood will be those that are well adapted to avoid/overcome these selection pressures, thus passing on desirable characteristics.
If the environment changes, selection pressures change, causing the variation being selected for to not be the modal variation, causing a directional selection.

Question 45

How does genetic drift lead to large changes in small populations?

Answer 45

Genetic drift is slight fluctuations in allele frequency. In small populations, these fluctuations are much more pronounced, as offspring have slightly different alleles to their parents, and there isn’t a large population to counteract slight variation. Thus, smaller populations have largely changing allele frequency and large changes in the population.

Question 46

How can populations be isolated, and what are the consequences?

Answer 46

Geographic barriers (river, mountain range)
Seasonal barriers (climate change throughout a year)
Reproductive mechanisms (incompatible genitals/breeding seasons/courtship behaviour)
If a population is isolated, different alleles will be eliminated or increased in each sub population, leading to the eventual development of a new species.

Question 47

What is the biological species concept?

Answer 47

The definition of a species ‘a group of similar organisms that can interbreed and produce fertile offspring and is reproductively isolated from other such groups’. This is problematic if the species in question doesn’t reproduce sexually or the male and female counterparts look significantly different.

Question 48

What is the phylogenetic species concept?

Answer 48

A group of organisms that have similar morphology (shape), physiology (biochemistry), embryology (stages of development) and behaviour and occupy the same ecological niche. Based on cladistics: where species are classified in groups dependent on their evolutionary ancestors. A clade is all the descendants of a common ancestor. Makes no distinguish between extinct and extant species, puts importance in quantitative (molecular) analysis, emphasis on evolution.

Question 49

Compare natural and artificial selection

Answer 49

Natural selection: mechanism for evolution; environment doing the selecting. Those organisms better adapted are more likely to survive to a reproductive age.
Artificial selection: humans select organisms with useful characteristics and allow them to breed (preventing those without the characteristics from breeding), humans have a significant effect of evolution of population/species.

Question 50

How has artificial selection brought about the modern dairy cow?

Answer 50

Cows have been repeatedly selected for high milk yields and bred together. Still goes on today, with milk yields being measured and recorded, ‘elite’ bulls and cows identified, given hormones to increase gamete production and fertilise in vitro, allowing elite bulls and cows to have more children than normal.

Question 51

How has artificial selection produced bread wheat?

Answer 51

Bread wheat is hexaploid: 6n = 42 (as opposed to 2n). This is due to artificial selection of a 2n plant, then breeding with another 2n plant to get a 4n plant (a mutation), then breeding with another 2n plant to get a 6n plant (a mutation). Bread wheat has 3 distinct genomes from 3 different species of wild wheat.