Inheritance, variation and evolution

What happens during interphase (before mitosis)?

DNA duplicates, organelles duplicate, the cell checks for errors and tries to fix them

What happens during the first stage of mitosis?

Prophase:

nuclear membrane disappears, DNA condenses + becomes visible, spindle fibres form

What happens in the 2nd stage of mitosis?

Metaphase:

chromosomes line up along the cell’s equator, spindle fibres attach to centromeres (middle part of chromosome)

What happens in the 3rd stage of mitosis?

Anaphase: 

spindle fibres pull chromatids to opposite poles

What happens in the 4th stage of mitosis?

Telophase:

nuclear membrane reforms (2 nuclei), spindle fibres disintegrate, DNA decondenses

What happens in cytokinesis (after mitosis)?

cytoplasm and cell membrane divide to give 2 genetically identical daughter cells.

What happens before meiosis 1?

DNA duplicates in diploid parent cell (in testes/ovaries)

What happens in the 1st stage of meiosis 1?

Prophase 1:

• nuclear membrane disappears, DNA condenses, spindle fibres form

• homologous pairs of chromosomes align

• crossing over occurs- homologous pairs swap sections of DNA

2nd stage of meiosis 1

Metaphase 1:

homologous chromosomes line up at the equator, spindle fibres attach

3rd stage of meiosis 1

Anaphase 1:

homologous chromosomes are pulled to opposite poles by the spindle fibres

4th stage of meiosis 1

Telophase 1:

spindle fibres disappear, nuclear membrane reforms

what happens in cytokinesis (after meiosis 1)?

cytoplasm and cell membrane divide to give 2 genetically non-identical haploid cells

1st stage of meiosis 2

prophase 2:

nuclear membrane disappears, spindle fibres form

2nd and 3rd stages of meiosis 2

metaphase 2:

chromosomes line up at the equator, spindle fibres attach at centromere

anaphase 2:

chromatids are pulled to opposite poles by spindle fibres

4th stage of meiosis 2

telophase 2:

nuclear membrane reforms, spindle fibres disappear, DNA decondenses

what happens in cytokinesis (after meiosis 2)?

• cytoplasm and cell membrane divide to give 4 genetically non-identical haploid daughter cells

• random fertilisation occurs (any sperm can join with any egg)

differences between mitosis and Meiosis

• mitosis happens in general body cells, meiosis in gametes

• mitosis is for growth/repair/development, meiosis for reproduction

• variation is not introduced in mitosis

• there are two divisions in meiosis (as opposed to 1)

• daughter cells are haploid not diploid in meiosis

• daughter cell has half the number of chromosomes as parent cell in meiosis

• daughter cells are genetically different to parent and each other in meiosis

allele

a version of a gene. can be dominant (e.g. A) or recessive (e.g. a)

• single copy of dominant masks recessive

• 2 copies of recessive needed to show recessive phenotype

homozygous vs heterozygous

homozygous contains 2 copies the same allele (either both dominant or both recessive)

heterozygous contains 1 copy of each allele (shows dominant characteristic)

Mendel’s pea plant experiment: why wasn’t Mendel originally believed?

• we did not have the understanding of genetics to appreciate his findings

• other members of the scientific community had not reviewed his work

what is cystic fibrosis?

a genetic disorder caused by a recessive allele which affects the cell membranes and prevents the movement of certain substances in and out of a cell. mucus is thicker and stickier than normal, making it difficult to breathe

what is polydactyly?

a condition caused by a dominant allele where babies are born with extra digits on their hands/feet.

how do you use parental phenotypes and genotypes to figure out the phenotypic ratio of the offspring?

by drawing a Punnett square

what is the sex chromosome for a female?

XX (fully homologous)

what is the sex chromosome for a male?

XY (not fully homologous)

what is embryo screening?

the process whereby a few cells are taken from an embryo produced by IVF checked for defective genes.

It does not alter the genetic sequence but can detect if the developing embryo is likely to develop a genetic disorder

how does embryo screening work? (4 steps)

1. DNA is isolated from the embryo cells

2. A fluorescent probe is mixed with the embryo DNA

3. the probe then binds to the DNA

4. UV light will glow/fluoresce if the probe binds to an allele that will likely cause a genetic disorder

pros of embryo screening (5)

• reduces overall suffering (fewer people with health issues)

• can save money (not a cheap process but saves for an individual as having a child with a genetic disorder is expensive)

• may identify a life-threatening condition

• if enough people are screened, some genetic disorders may be removed from the population

• carefully monitored/controlled in the UK

cons of embryo screening (7)

• creates stigma, implies that people with genetic disorders are undesirable

• could lead to future screening for other traits e.g. eye colour

• embryo not used may be destroyed

• process costs a lot of money

• embryos with non-life-threatening conditions may be terminated

• if insurance companies obtained this information, they could charge more is you had a genetic disorder

• reduces diversity as it reduces the number of alleles in the population

what is evolution

a change in the inherited characteristics of a population over time through a process of natural selection. it may result in the formation of a new species. the theory of evolution states that today’s life evolved from simple life forms 3 billion years ago.

Lamarck

acquired characteristics (check book for model answers)

Darwin

belief/theory: inherited characteristics, natural selection (model answers in book)

ship: HMS Beagle, book: On the Origin of Species

why was Darwin’s theory not accepted widely originally?

it went against common religious beliefs/ the idea of God

Wallace

warning colouration and theory of speciation (model answer in book)

what is warning colouration?

species evolve to look like a more dangerous species for protection (also known as mimicry)