4b diversity and selection

diploid cells

diploid cells contain two of each chromosomes (2n). the chromosomes are the same size and have the same genes but can have different alleles

humans have 23 homologous pairs of matching chromosomes. therefore the diploid number for humans is 46

gametes in fertilisation

gametes are the sperm cells in males and egg cells in females. they have a haploid number of chromosomes (n)

two gametes join together at fertilisation to form a zygote. half of the chromosomes are from the father, and half from the mother. zygotes are diploid

fertilisation and how it increases genetic diversity

a haploid sperm fuses with a haploid egg, making a diploid cell.

during sexual reproduction, fertilisation is random, meaning zygotes with different combinations of chromosomes are made. this increases genetic diversity within a species

meiosis definition

meiosis is a type of cell division which happens in the reporductive organs of multicellular, eukaryotic organisms. the cells begin diploid and finish haploid. it is used to create gametes

the stages before meiosis

1. dna unravels and replicates so there are two copies of each cheomosome

2. the dna condenses to form double armed chromosomes, made from two sister chromatids. they are joined by a centromere

meiosis 1

3. the chromosomes arrange themselves into homologous pairs

4. the pairs are separated, halving the chromosome number

meiosis 2

5. the paire of sister chromatids that make up each chromosome are divided

6. four genetically different haploid cells are produced

crossing over (chiasmata)

during meiosis 1, homologous chromosomes come together and pair up. thr chromatids twist around each other and bits of chromatids swap over. the chromosomes still contain the same genes but now have a different combination of alleles

independent segregation

each homologous pair of chromosomes in your cells is made up of maternal and one paternal chromosome. when the homologous pairs are separated in meiosis 1, the chromosomes that end up in each daughter cell is random. therefore, each daughter cell has a different combination of maternal and paternal chromosomes

base substitution

one base is substituted with another

ATGCCT → ATTCCT

not all substitutions with change the amino acid sequence

base deletion

one base is deleted

ATGCCT → ATCCT

it will shift all of the bases to the left, changing the amino acid sequence

mutagenic agents

mutagenic agents increase rate of genetic mutation, e.g. uv radiation, ionising radiation

chromosome non-disjunction

a chromosome fails to separate properly during meiosis 1, so one cell gets an extra copy of it. when the gamete with an extra chromosome fuses with another, the zygote has three copies of the chromosome

genetic diversity

the number of different alleles in genes in a species or population

genetic bottlenecks

an event that causes a big reduction in a population, reducing the number of different alleles in a gene pool and reducing genetic diversity

how natural selection works

1. not all individuals are as likely to reproduce as each other. individuals that have an allele that increases their chances of survival will reproduce more

2. so a greater proportion of the next generation inherits the beneficial allele

3. so they ard more likely to survive, and pass on their genes. the freqeuncy of the allele continues increasing

4. over generations this leads to evolution

behavioural adaptations

ways an organism acts to increase its chance of survival

physiological adaptations

processes inside an organism’s body that increase its chance of survival

anatomical adaptations

structual features of an organisms body that increase its chance of survival

directional selection

individuals with alleles for characteristics of an extreme type are more likely to survive and reproduce

stabilising selection

individuals with alleles for characteristics towards the middle of the range are more likely to survive and reproduce

required practical 6

testing the effects of antibiotics

1. the bacteria are grown in a liquid broth

2. using a sterile pipette, bacteria is transferred from the broth to an agar plate

3. sterile forceps are used to place paper discs soaked with different antibiotics onto the plate

4. a lid is taped onto the petri disc. it is placed upside down and incubated at around 25°C for 48 hours

5. anywhere the bacteria can’t grow is seen as a clear patch, called the inhibition zone. the larger the zone, the better the antibiotic

required practical 6

aseptic techniques

• disinfect work surfaces and put contaminated utensils in a beaker of disinfectant to minimise contamination

• use sterile equipment and discard safely after use

• work near a bunsen flame so microbes in the air are drawn away from the plate

• keep the agar plate lid on as much as possible to reduce airborne microbes getting in

• flame the neck of the glass container of broth after it’s opened and before it’s closed to prevent unwanted organisms falling in