Mitosis and Meiosis [Intro, mutations, bioethics]

What is the difference between the structure of chromatin and chromosomes?

Chromatin → Uncondensed form of DNA

Chromosomes → Condensed form of DNA, tightly coiled

What are sister chromatids?

Chromatids that contain identical DNA molecules as they are replicated from the DNA molecule

What is the difference between diploid and haploid?

Diploid

• 2n; 2 complete sets of chromosomes

• Exist as homologous pairs in many somatic cells

Haploid

• n; 1 complete set of chromosomes

• Exist in gametes

*Used to describe a nucleus, cell or organism

How is a diploid zygote formed and what happens to it after?

Fusion of a haploid sperm and egg during fertilization to form diploid zygote → Mitosis to form cells that are genetically identical to the original zygote → Stimulated to differentiate into specialised cells

What are homologous chromosomes?

Two chromosomes having the same size, shape, centromere position, staining pattern and position of genes

What are the characteristics of homologous chromosomes?

• Same genes at corresponding loci, but may have different alleles in the same locus

• One homologue from a male, one from a female

What are alleles?

Alternative forms of a gene that occurs[y the same loci on homologues

What happens during interphase in general?

• Period of growth and synthesis

• Cell produces many materials and organelles required for its functions

What happens during G1 of interphase?

Intensive cellular synthesis of organelles, RNA, protein, ATP etc.

What happens during S of interphase?

DNA replication → DNA content of cell doubles

What happens during G2 phase of interphase?

Intensive cellular synthesis of organelles, spindle proteins and ATP etc.

What happens by the end of interphase? (4)

• DNA is replicated

• Nucleus is bound by nuclear envelope

• Nucleolus is present

• Single microtubule organizing centre duplicated to form 2 centers, each containing a pair of centrioles

What is aneuploidy?

A genetic disorder where the cell does not have a chromosome number that is a multiple of the haploid number i.e. chromosomes are present in extra or fewer copies than the wild type

How does aneuploidy come about?

• Anaphase I → Homologous chromosomes do not move properly to opposite poles

• Anaphase II → Sister chromatids fail to separate properly to opposite poles

• Results in one gamete receiving 2 of the same type of chromosome and the other gamete receiving no copy → Mitosis subsequently transmits the mutation to all cells of the embryo that arose from this mutant cell

• Can also occur during mitosis

What are the 2 kinds of aneuploidy?

• Chromosome in triplicate (2n+1) → trisomic cell

• Missing a chromosome (2n-1) → monosomic cell

What is an example of aneuploidy?

Down syndrome: Extra chromosome 21 → 47 chromosomes

• Usually from non-disjunction in meiosis I

• Symptoms include physical development and intellectual impairment, behavioral problems

What are the principles of bioethics?

• Respect for persons

• Maximize benefits and minimize harms

• Justice

How does chromosome mutation occur due to pregnancy?

• Errors during formation of gametes (meiosis) or during cell division (mitosis) → Foetus has incorrect number or structure of chromosomes

• Risk of chromosomal mutations in pregnancy increase with maternal age

What types of pre-natal screening are there for pregnancy?

• Non-genetic methods e.g. soundwaves in ultrasound tests

• Genetic methods e.g. obtaining foetal DNA through amniocentesis i.e. obtaining amniotic fluid

What are arguments against genetic screening tests during pregnancy for parents? (4)

• Risk of pregnancy loss (when harvesting foetal tissue)

• False results may occur

• Pressure on mother

• Stress of decision making e.g. facing arguments to terminate the pregnancy

What are arguments against genetic screening tests during pregnancy for the unborn child?

Promotes discrimination against children with genetic condition → Mothers terminate pregnancy for children who are less than perfect

What are arguments against genetic screening tests during pregnancy for society? (2)

• Unequal distribution of resources: No publicly-funded prenatal screening in many healthcare systems → Only for patients who can afford

• Legal issues: Genetic information may be used in the future for law enforcement

What are arguments for genetic screening tests during pregnancy for parents? (4)

• Protects mother’s physical health should the pregnancy endanger her life

• Certain tests are relatively safe → Not all carry a high risk of pregnancy loss

• Mother’s right is respected as these prenatal tests are optional

• Allows for counseling and preparation for parenthood

What are arguments for genetic screening tests during pregnancy for the unborn child? (2)

• Some genetic conditions may be corrected at foetal stage

• Follow-up diagnosis can be arranged if necessary before the final one

What are arguments for genetic screening tests during pregnancy for society?

Regulation can be done → Prenatal tests only for the medical the unborn child and not adult-onset conditions