Eduqas A level Biology Component 2

Journey of the sperm

1. The sperm are released from the epididymis and travel along the vas deferens out of the penis through the urethra

2. Spermatozoa are deposited at the top of the vagina and swim through the cervix and along the lining of the uterus, into the oviduct where they meet the secondary oocyte

Fertilisation

1. It occurs at the beginning of the fallopian tube

2. Spermatozoa can only fertilise an ovum after a process called capacitation

3. Capacitation of the sperm occurs as it moves through the fluid in the uterus

4. It causes a change in the membrane covering the acrosome, so increasing the chance of binding the sperm to the secondary oocyte

Capacitation

enzymes in the uterus cause the:

• Removal of glycoproteins and plasma proteins from the surface of the sperm

• Cholesterol is also lost from the cell surface membrane around the sperm head, weakening the membrane

• The membrane becomes more permeable to Ca+ which increase the beating activity of the sperm tail and promotes the acrosome reaction

• During capacitation, the acrosome membrane fuses with the cell surface membrane, a process which starts the release of acrosomal hydrolytic enzymes such as proteases

Content of sperm head

haploid nucleus and an acrosome which contains enzymes used in fertilisation

Function of sperm flagellum

makes lashing movements that move the sperm

Function of FSH

• Acts on the ovaries

• Stimulates the development of a mature graafian follicle

• Stimulates the outer fibrous layer of the graafian follicle to produce oestrogen

What happens if there is no implantation?

• The falling concentrations of FSH and LH cause the corpus luteum to break down

• Breakdown of the corpus luteum means progesterone production declines causing the endometrium to shed

• As oestrogen is low, it no longer inhibits FSH production, so the menstrual cycle restarts

Describe the follicular phase

1. The pituitary gland secretes FSH

2. At the ovary, this causes the follicle cells surrounding a primary oocyte to divide. The developing follicle cells begin to secrete oestrogen

3. This stimulates the growth of the endometrium

4. Increasing levels of oestrogen inhibit FSH but increases LH release from the pituitary

5. The increasing LH has a positive feedback on FSH and its levels pick up again

6. The developing follicle progresses to a graafian follicle and peak levels result in ovulation

Describe the secretory phase

1. The remains of the graafian follicle develop into the corpus luteum

2. This secretes oestrogen and a large amount of progesterone

3. Progesterone continues to stimulate the growth of the endometrium

4. High levels of progesterone and oestrogen inhibit FSH and LH

5. Without FSH and LH, the corpus luteum begins to shrivel

6. Progesterone and oestrogen levels fall and the endometrium is shed

7. FSH is no longer inhibited and the cycle begins again

What is the corolla?

a collective term for all the petals

What is a receptacle?

the basal portion of a flower to which all floral parts are attached

What is the function of the filament?

contains vascular tissue that supports the anther. it also transports food materials necessary for the formation of the pollen grains

What is the carpel?

consists of the stigma, style and ovary

What is the function of the stigma?

its sticky because it receives pollen from the anther

What is the style?

the organ through which pollen grows a tube down

What is the function of the ovary?

the fertile part of the carpel, containing the ovules and developing into the fruit. meiosis occurs in the ovary to produce haploid ovules

Describe the formation of pollen

1. Each pollen sac is filled with diploid cells containing large nuclei. As the anther grows, each of these cells goes through 2 meiotic divisions, forming a tetrad. These cells are called microspores

2. Each one of these microspores eventually becomes a pollen grain

3. Inside the pollen grain, the haploid nucleus undergoes mitosis to produce 2 nuclei, a generative nucleus and a tube nucleus. The generative nucleus later gives rise to the 2 male nuclei

4. When the pollen is mature, the outer layers of the anther dry out. The anther splits open along its length and opens out. This is called dehiscence and it releases the pollen grain so pollination can occur

What is the function of LH?

• Induces ovulation on day 14 - causes the graafian follicle at the surface of the ovary to release secondary oocytes

• Remains of the graafian follicle converts to the corpus luteum

• The corpus luteum secretes oestrogen and progesterone

• Oestrogen and progesterone inhibit the further secretion of FSH and LH so their concentrations decrease

What are the sepals?

outermost ring of structure of a flower, usually green and the main function is to protect the flower when it is in bud

What is the calyx?

collective term for all the sepals

What is the stamen?

consists of 2 parts: anther and filament

What is the function of the ovule?

made inside the ovary and contains an egg cell formed by a process involving meiosis. after fertilisation, the ovules eventually form seeds

Describe the structure of the ovary

• Each ovary contains one or more ovules

• The integuments are the 2 walls of the ovule

• There is a small opening in the walls called a micropyle. This is where the pollen tube will enter. The nucleus are cells that provide nutrition for the growth of the ovule

Describe the development of the female gamete in plants

1. In the ovule, a mother diploid cell divides by meiosis to form 4 haploid cells (megaspore). 3 of the cells degenerate and 1 megaspore

2. The haploid nucleus of the surviving megaspore undergoes 3 mitotic divisions

3. 8 haploid nuclei are in the embryonic sac and are surrounded by the nucellus which is a layer of cells that provide nutrients

4. 2 of the haploid nuclei (polar nuclei) fuse to make diploid polar nuclei

What does the embryo sac in a mature ovule contain?

3 antipodals (haploid)

2 synergids (haploid)

1 egg/oosphere nucleus (haploid)

1 polar nucleus (diploid)

Describe double fertilisation

1. If a pollen lands on the stigma of a plant of the same species, the stigma produces a sugary solution in which the pollen grain germinates producing a pollen tube

2. The pollen tubes secretes enzymes (cellulases and proteases) as it grows, digesting its way through the tissues of the style. It also gains nutrients from the digested products. The growth occurs down the style, up a gradient of chemo-attractants e.g. GABA form the ovule. At the tip of the pollen tube are 3 nuclei

3. The pollen tube nucleus precedes the other 2 and controls the growth of the pollen tube

4. Behind the tube nucleus are 2 male haploid nuclei which have originated from the mitotic division of the generative nucleus

5. When the pollen tube reaches the ovary, it enters the ovule through the micropyle

6. The pollen tube nucleus disintegrates and the tip of the pollen tube bursts

7. The 2 male gametes are released into the embryo sac

8. 1 male gamete fuses with the egg nucleus to forma diploid zygote

9. The other male gamete fuses with both polar nuclei at the centre of the embryo sac to form a triploid endosperm nucleus. A double fertilisation occurs, a process that only occurs in flowering plants

10. The zygote eventually develops into an embryo and grows into a new plant

What is pollination?

the transfer of pollen from the anther to the stigma

What are the advantages of cross-pollination?

• Outbreeding promotes genetic variation

• Prevent inbreeding

• More evolutionary potential

• Plants can withstand environmental change

• Not all wiped out by disease

• Reduces the chance of producing harmful allele combination

• Recessive alleles are less likely to be expressed

• Utilises entire gene pool

What are the advantages of self-pollination?

it can preserve good genomes which may be suited to a stable environment

What are the disadvantages of cross-pollination?

• Prevents purity of gene pool

• Hard to guarantee

What happens after fertilisation in plants?

• Fertilised ovary becomes the fruit

• Ovule develops into the seed

• Integuments becomes the seed coat/testa

• Zygote becomes an embryo

What makes up a plant embryo?

consists of the radicle which gives rise to the root, and a plumule which is the growing part of the shoot; the part containing these if the hypocotyl

What is the food store in a plant?

the reserve of food which supplies the embryo and growing plant until it is able to make its own food. it may be in the cotyledon or endosperm

What is germination?

biochemical and physiological processes through which a seed becomes a photosynthesising plant

Describe the germination of a broad bean

• The food reserves must be broken down into simple soluble substances which dissolve in water and are then transported to the plumule and radicle

• Water is taken up rapidly by the seed, causing the tissues to swell as well as mobilising the enzymes

• Amylase hydrolyse starch in the endosperm into maltose which is transported to the growing arts for respiration

• The testa ruptures as the radicle pushes its way through, the radicle grows down and plumule up

• The cotyledons remain below ground

• The plumule is bent over as it pushes through the soil which protects the tip from being damaged

• When the plumule emerges, it unfurls and begins to photosynthesise

What is the function of the anther?

the fertile portion of the stamen, the part in which pollen is produced, usually consisting of 4 pollen sacs. the pollen sacs eventually split to release the pollen

What are the features of wind pollinated plants?

• Pollen grains are small and light

• Large quantities of smooth pollen

• Petals are small and green as no need to attract insects

• Anthers are exposed to the wind so pollen can easily be blown away

• Stigmas are large and feathery to catch pollen carried by the wind

• No scent or nectar

What is genetic drift?

an evolutionary mechanism that occurs in small populations that changes the gene pool

What is a bottleneck?

when the entire species is wiped out except for a small group of survivors. the allele frequencies in the survivors determines the allele frequencies in the population

What is allopatric speciation?

populations that evolve independently of each other due to geographical isolation tot he environmental conditions where they live. the 2 populations may start to diverge and develop other mechanisms that will keep them reproductively isolated should they meet in the future

What is sympatric speciation?

when organisms inhabiting the same area become reproductively isolated into 2 groups when there are no physical barriers. this can be due to temporal/seasonal isolation, behavioural isolation, mechanical isolation, gamete isolation, hybrid inviability and hybrid sterility

What is the phylogenetic species concept?

a group of organisms that have a similar morphology, physiology, embryology and behaviour and occupy the same ecological niche

What is gibberellin?

• Embryo produces gibberellin which turns on a gene in the aleurone layer which codes for amylase

• The amylase produced breaks down starch in the endosperm into maltose

• The maltose is used as fuel by the growing embryo

What is an allele?

different form of the same gene

What is genotype?

sets of genes responsible for a particular trait

What is monohybrid inheritance?

the inheritance of a single characteristic controlled by one gene

What is sex linkage?

• This means that XY males only have one copy of the gene

• If it's a recessive allele, then it will be expressed in the phenotype whereas in XX people, there is a second gene which if dominant will make them a carrier and wont be expressed

• Sex linked gene cant be passed from father to son as the X chromosome comes from the mother

What is autosomal linkage?

• If 2 genes are close together on the same chromosome, then they're likely to be more common in the parental combination and are said to be linked. The linked genes are inherited together and pass into the gametes together

• This may cause statistically significant variation between the observed results and the expected

• This can be tested using chi squared

What is a mutation?

a change in the amount, arrangement or structure of dna

most occur during crossing over in prophase 1 and non-disjunction during anaphase 1 and 2

What increases the mutation rate?

• Ionising radiation: x-rays, gamma radiation and uv light

• Chemicals such as polycyclic hydrocarbons etc,

• Biological agents: some bacteria such as helicobacter and viruses

What are the types of mutation?

gene and chromosome mutations

What are geen mutations?

changes ina single gene

takes place during DNA replication

can involved one or more nucleotides

may be done due to copying errors, chemicals, viruses etc

What are the causes of gene mutations?

a change in one nitrogen base in a DNA base sequence:

1. Substitution - a different base is incorporated

2. Addition - a base is added

3. Deletion - a base is deleted

4. Duplication - some bases incorporated twice

5. Inversion - adjacent bases on the same DNA strand exchange position

What is an example of deletion?

cystic fribrosis: one codon is omitted

What is an example of substitution?

sickle cell disease: nucleotide substitution

occurs in the haemoglobin gene

What causes sickle cell anaemia?

a single base substitution in the gene for beta haemoglobin with CTT replaced by CAT

This causes the 6th amino acid Glutamic acid to be replaced by valine

this change in the primary structure causes the secondary, tertiary and quarternary structure of the haemoglobin to change

Describe sickle cell anaemia

the haemoglobin molecule becomes insoluble at low oxygen tensiosn and it tends to stick to other haemoglobin molecules to form long fibres

the fibres distort the blood cells, making them sickle shaped

the mutated haemoglobin is poor at binding to oxygen

in a homozygous individual, all molecules are affected and the phenotype is the potentialy fatal 'sickle cell anaemia'

ina heterozygous individua;, half of the molecules are affected and the persons blood O2 level is reduced by 20% - 'sickle cell trait'

What are chromosomal mutations?

mutations that cause a change in the number and structure of whole chromosomes. they are most likely to occur during meiosis

Explain chromosomal mutations caused by changes in the structure

during prophase 1, errors happen when homologous chromosomes exchange genes at the chiasmata. homologous chromosomes might end up with a different gene sequence due to:

isolation of genes

number of genes

What causes down syndrome?

non-disjunction of chromosome 21 during anaphase 1 results in oocytes with na extra chromosome. daughte cells wiht no chromosome 21 die, but those with 2 copies survive and may be fertilised to produce a viable embryo containing 47 chromosomes

Explain chromosomal mutations caused by changes in sets of chromosomes

cells wiht several sets of chromosomes are described as polyploidy

an error in meiosis may result in a gamete recieving 2 sets of chromosomes

if 2 diploid gametes fuse, a tetraploid (4n) is produced

endomitosis is the repliccation of chromosomes that is not followed by cytokinesis. if this happens ina n early embryo, 4 sets of chromosomes are incorporated intot he nucleus and tetraploid cells are produced

What causes cancer?

the cause is mutations of a gene which would normally control cell division:

tumour supressor genes mutate so there's a lack of proteins that stop cell division

excess of proteins whihc stimulate cell division

lack of proteins involved in the normal mechanism of cell death

What are mutated genes called?

oncogenes

What controls gene expression?

the environment can alter the expression of genes by affecting how they are transcribed during protein synthesis, without changing the DNA base sequence

the changes are epigenetics and happen by 2 mechanisms:

DNA methylation

histone modification

What is DNA methylation?

cytosine have methyl or hydroxymethyl group

the addition of a methyl group to cytosine prevents cytosine and guanine bases from being transcribed

What is histone modification?

in eukaryotes, DNA is wrapped histone proteins and forms nucleosome

chromatin can be highly or loosely packed and correlated to the gene expression levels

modifications in gistone proteins affects the structure of chromatin

highly packed chromatin menas there is low gene expression as less accessible to enzymes and vice versa

What is variation?

the differences between organisms of the same species

What are sex-linked traits

genes for these traits are located only on the X chromosome

sex linked recessive phenotypes are more common in men than women

What are examples of sex-linked traits?

1. red-green colour blindness - due to a recessive allele which leas to a failure to produce functional cone cell pigments

2. haemophilia - a recessive allele leads to the failure to produce a blood clotting protein

What are the reasons for variation?

• Different genotype

• Same genotype but different epigenetic modifications

• Have different environments

What is continuous variation?

controlled by a number of genes

described as polygenic

character shows a gradiation form one extreme to another

environemnt has a consideable influence on gene expression

e.g. height

What are monocotyledons?

only one cotyledon

What are dicotyledons?

2 cotyledons

What is a monocot fruit?

zygote only has one cotyledon

endosperm is retained and is the food source once the seed germinates

What is the testa?

tough protective outer layer. it is chemically resistant and so seeds can survive adverse chemical condittions. it protects the embryo

What are the hormones involved in the menstrual cycle?

FSH

LH

Oestrogen

Progesterone

What is the functio of the petal?

inner sterile whorl of a flower, often coloured to attract insects

What rea the features of insect pollinated plants?

• Small quantity of sticky sculptured pollen for hooking not insects body

• Produces large pollen grains

• Anther and stigma within the flower

• Large and brightly coloured petals

• Sticky stigma to collect pollen

• Nectar and scent present

What is self-pollination?

occurs when pollen falls from the anther to the stigma of the same flower. it results in inbreeding

What rea the disadvantages of self-pollination?

not desirable as it reduces variation

a greater chance of 2 undesirable recessive alleles being brought together

What is cross pollination?

the transfer of pollen from the anther of one flower to the maturing stigma of another flower on another plant of the same species

What are the ways in which seeds can be dispersed?

wind

water

mechanical/bursting

transport by animals

carrying - attached to animal's coats

rolling

What is the founder effect?

when the population is isolated on an island or new habitat, the founder members are a small sample of the original population, by chance, they may have a very different allele frequency to the original population and if the population size remians small, it may undergo genetic drift

Describe natural selection

1. In any population, variation is due to mutations

2. Individuals in a population have the potential to produce large numbers of offspring

3. The number of adults however remains constant from one generation to the next

4. There is a struggle for survival as only the 'fittest' survive

5. The individuals that survive and reproduce pass on their characteristics to their offspring

6. Over time, a group of individuals that once belonged to the same species may give rise to 2 groups that are significantly distant to belong to separate species

7. If the environment changes, natural selection continues

What is hybrid sterility?

when the hybrid is viable but sterile in adulthood

What is hybrid inviability?

despite fertilisation occuring, debelopment of the embryo may not occur. this may be because the chromosomes dont match or there is an odd number that cant divide in meiosis

What is hybrid breakdown?

some F1 hybrids are fertile, but their F2 is sterile

What is a gene?

segment of DNA that codes for a specific polypeptide chain

How would you perform a test cross (back cross)?

cross an individual with a dominant phenotype with a homozygous recessive individual

if the unknown is homozygous, all offspring will have a dominant phenotype,

if the unknown is heterozygous, half will show the recessive phenotype and the other half dominant

What is co-dominance?

both alleles are dominant, they are both expressed equally in the phenotype

the heterozygotes show a phenotype intermediate of the 2 genes

What are multiple alleles?

3 or more forms of the same gene that codes for a single trait

e.g. blood type

What is a frame shift mutation?

inserting or deleting one or more nucleotides so that it changes hte 'reading frame' so proteins are built incorrectly i.e. non-functional proteins. if the protein is an enzyme, the structure and shape of the active site could be different

What is phenotype?

physical expression or characteristics of a trait

What are mutagens that cause cancer called?

carcinogens

What is cancer?

uncontrolled divison of cells and the production of a tumour.

in malignant tumours, cells may break off and be carried in the blood to form secondary tumours

What is discontinous variation?

controlled by a single gene

describe as monogenic

no internediate forms

characteristics are clear cut and easy to tell apart

environemnt has no influence on gene expression

e.g. blood type

What is heritable variation?

results from genetic changes due to sexual reproduction:

the mixing of 2 different parental genotype

random assortment of homologous chromosomes in metaphase 1

crossing over between homologous chromosomes in prophase 1

these changes establish new allele combinations, onl mutations generate long-lasting effects

What is non-heritable variation?

environmental influences that determine phenotypic variation

in humans - diet and exercise

in plants - temperature, light and ions

if organisms of identical genotypes are subject to different environmental conditions, then they will show variety

What is selection?

the process by which organisms better adapted to their environment survive and breed, while those less well adapted fail to do so. the better adapted organisms are more likely to pass on their characteristics to succeeding generations

What is stabilising selection?

if an organism has a favourable phenotype, its more likely to produce offspring similar to itself. if the environment is stable, then the extreme variations tend to be eliminated