Topic 3: 3.6 to 3.10

What is the function of the haploid nucleus in a sperm cell?

It contains half the number of chromosomes to restore the diploid number upon fertilisation.

How many copies of each gene does a sperm cell contain?

One copy of each gene; though different sperm may carry different alleles.

What is the role of the sperm tail?

It allows movement towards the egg cell.

What is the function of the sperm middle piece?

It contains many mitochondria to provide energy for the movement of the flagellum.

What is the acrosome and its function?

A structure in the sperm head containing hydrolytic enzymes to break down the zona pellucida.

How does the egg cell cytoplasm support development?

It contains proteins and lipids that act as food reserves for the developing embryo.

What is the zona pellucida?

A jelly-like layer surrounding the egg cell.

What are cortical granules?

Lysosomes in the egg that release chemicals to harden the zona pellucida after fertilisation.

What is the purpose of the cortical reaction?

To harden the zona pellucida and prevent polyspermy.

Outline the cortical reaction

Events after fertilisation:

• Lysosomes in egg cell migrate to egg cell membrane

• Lysosomes fuse with membrane (exocytosis) and release chemicals

into zona pellucida/jelly layer – the cortical reaction

• Zona pellucida hardens and this prevents polyspermy

Define fertilisation.

The fusion of haploid gametes to form a diploid zygote.

What triggers the acrosome reaction?

Chemicals released from the ovum when the sperm cell reaches it.

What happens during the acrosome reaction?

Sperm cell reaches ovum

• Chemicals released from ovum trigger the acrosome reaction

• Acrosome swells and fuses with sperm cell surface membrane

• Hydrolytic enzymes released from acrosome

• Enzymes digest through follicle cells and zona pellucida/jelly layer

• Sperm cell membrane fuses with egg cell membrane

• Sperm nucleus enters egg cell

• Nuclei fuse to give diploid zygote (fertilisation)

How does the sperm nucleus enter the egg?

Enzymes digest the follicle cells and zona pellucida; then the sperm and egg membranes fuse.

What is a gene locus?

The specific position of a gene on a chromosome.

What is genetic linkage?

When two or more genes have loci on the same chromosome and are not separated unless crossing over occurs.

What is the difference between close and weak linkage?

Close linkage involves genes physically near each other (less likely to cross over); weak linkage involves genes far apart (more likely to cross over).

What is sex linkage?

Genes located on the sex chromosomes that are passed on with the genes determining sex.

Why are recessive sex-linked traits more common in males?

Males (XY) only have one X chromosome; so a single recessive allele will always be expressed.

What is autosomal linkage?

When genes are located on the same chromosome; excluding the sex chromosomes.

How is DNA organised into a condensed chromosome?

DNA winds around histone proteins to form chromatin; which then coils and organises on protein scaffolds.

What does a chromosome consist of during cell division?

• two identical sister chromatids joined at a centromere

What are the products of meiosis?

Four haploid (n) daughter cells that show genetic variation.

How does meiosis lead to genetic variation?

• independent assortment

• Crossing over

What occurs during Meiosis I?

The chromosome number is halved and variation is introduced via crossing over and independent assortment.

How does independent assortment create variation?

It produces new combinations of maternal and paternal chromosomes.

• 2^n combos where n is the number of chromosome pairs

What happens during crossing over?

Homologous chromosomes come together as bivalents and non-sister chromatids break and rejoin, exchanging different DNA sections.

Different combos of alleles of chromosomes produced.

Why is there no crossover between X and Y chromosomes?

Because they are not homologous; as the Y chromosome is shorter.

What are the three main purposes of mitosis?

Growth; repair/replacement of cells; and asexual reproduction.

What happens in G1?

• sub cellular structures are replicated

• Cell grows in size

What happens in S phase?

DNA is replicated

What happens in G2?

cell checks DNA for mutations and repairs any errors

What happens during Prophase of mitosis?

DNA condenses making chromosomes visible; centrioles migrate to poles to lay down spindle fibres; and the nuclear envelope/nucleolus break down.

What happens during Metaphase of mitosis?

Chromosomes line up at the equator and spindle fibres attach to the centromeres.

What occurs during Anaphase of mitosis?

Spindle fibres contract and pull individual sister chromatids to opposite poles.

What occurs during Telophase of mitosis?

Chromosomes decondense; nuclear envelopes and nucleoli reform; resulting in two nuclei and two nucleoli reformed.

A full set of chromosomes at each of the opposite poles.

What is cytokinesis?

The division of the cytoplasm amd cell membrane to form two genetically identical daughter cells (clones).

What is the purpose of soaking a root tip in HCl during a squash?

To break down the middle lamella and separate the cells.

What are the three stages of the cell cycle?

Interphase; mitosis (nuclear division); and cytokinesis (cytoplasmic division).

What happens during the S phase of interphase?

DNA is replicated.