Mammalian Fertilisation

Gametes

Process

• Sperm swims through the uterus.
• If sperm enters at about the time of ovulation, sperm may meet the ovum in the oviduct.
• The sperm are attracted to the ovum by chemicals released from it.
• Once the sperm reaches the ovum, the chemicals released from surrounding cells trigger the acrosome reaction in the sperm.
• The acrosome swells, fusing with the sperm cell surface membrane.
• Digestive enzymes in the acrosome are released.
• The enzymes digest through the follicle cells and zona pellucida surrounding the ovum.
• This triggers the cortical reaction; the egg cell releases the contents of vesicles called cortical granules into the space between the cell membrane and the zona pellucida.
• These chemicals cause the zona pellucida to thicken and prevent entry from other sperm (polyspermy)
• The sperm nucleus that enters the egg fuses with egg nucleus to produce a fertilised egg, a zygote.

Protein Transport

Process

• DNA is transcribed to form mRNA
• The mRNA leaves the nucleus and moves onto one of the ribosomes on the rough endoplasmic reticulum (rER)
• The proteins are made at the ribosomes on the rER using the mRNA
• These new proteins are folded and processed as they move through the rER, assuming 3D shape en route
• Vesicles xontaining the protein are pinched off the rER
• These vesicles fuse to form the flattened sacs of the Golgi apparatus
• The proteins are modified within the Golgi apparatus
• Vesicles are pinched off the Gogli apparatus that contain the modified protein
• The vesicles fuse with the cell surface membrane releasing the protein

Helpful Video

Animal Cell (Eukaryote)

Mitochondria

• Inner membrane is folded to form finger-like projections called cristae.
• The mitochondria are the site of the later stages of aerobic respiration

Nucleus

• Enclosed by an envelope made up of 2 membranes perforated by pores
• Contains chromosomes and a nucleolus
• The DNA in chromosomes contains genes that control protein synthesis

Nucleolus

• A dense body within the nucleus where ribosomes are made

Rough Endoplasmic Reticulum (rER)

• A system interconnected, membrane-bound, flattened sacs
• Ribosomes are attached to the outer surface
• Proteins made by these ribosomes are transported through the ER to other parts of the cell

Ribosomes

• Made of RNA and protein
• Are found free in the cytoplasm or attached to endoplasmic reticulum
• The site of protein synthesis

Cell Surface Membrane

• Phospholipid bilayer containing proteins and other molecules forming a partially permeable barrier

Smooth Endoplasmic Reticulum (sER)

• Similar to rER but with no ribosomes attached
• Makes lipids and steroids, e.g. reproductive hormones

Golgi Apparatus

• Stacks of flattened, membrane-bound sacs formed by fusion of vesicles from the ER
• Modifies proteins and packages them in vesicles for transport

Lysosomes

• Spherical sacs containing digestive enzymes and bound by a single membrane
• Involved in the breakdown of unwanted structures within the cell
• Involved in destruction of whole cells when old cells are to be replaced or during development

Centrioles

• Hollow cylinders made up of a ring of 9 protein microtubules
• Involved in the formation of the spindle during nuclear division and in transport within the cell cytoplasm

Prokaryotic Cell (Bacteria)

Function
Flagellum - hollow, cylindrical, thread-like structure that rotates to move the cell.
Pili - thin, protein tubes that allow bacteria to adhere to surfaces
Capsule - slimy layer on surface for protection and to prevent dehydration
Infolding of cell membrane  - site of respiration

Unit 2: Development, Plants & The Environment

The topics that are covered in the specification are:

• Prokaryotic Cell
• Animal Cell (Eukaryote)
• Protein Transport
• Mammalian Fertilisation

Protein Synthesis

Transcription

• This process takes place in the nucleus of a eukaryotic cell.
• The DNA double helix unwinds; the enzyme helicase is used to break the hydrogen bonds between the bases, allowing the 2 strands to partly separate.
• The sequence of 1 of the strands becomes the template / antisense strand and is used in the production of mRNA.
• mRNA is built from free RNA nucleotides which line up alongside the DNA template strand.
• Due to complementary base pairing e.g. U pairs with A and C with G, the order of bases on the DNA exactly determines the order of bases on the RNA strand.
• Every triplet code on the DNA creates a complementary codon on the mRNA.
• The individual mononucleotides are joined together by phosphodiester bonds.
• The completed mRNA molecules now leave the nucleus through pores in the nuclear envelope into the cytoplasm.
• The cytoplasm is where the 2nd stage of protein synthesis, translation, takes place.

Translation

• This takes place on ribosome's on endoplasmic reticulum in the cytoplasm.
• The mRNA attaches to the ribosome.
• A transfer RNA (tRNA) molecules carrying an amino acid molecule has 3 bases called an anticodon.
• These pair with complementary bases on the mRNA codon.
• The ribosome moves along the mRNA as it pairs up with the tRNA anticodons until it reaches the stop codon.
• Then the amino acids that the tRNA carry are joined by peptide bonds formed in condensation reactions to produce a polypeptide chain.

DNA Replication

• The DNA unwinds as the enzymes, helicase, causes the hydrogen bonds between the strands to break.
• This casues the polynucleotide chains to separate, and expose the bases.
• Each chain now acts as a template.
• Free nucleotide molecules (mRNA) in the nucleus position along the exposed bases on both strands.
• Hydrogen bonds form between the complementary bases on the template strand and the nucleotide molecules.
• The enzyme DNA polymerase joins the adjacent nucleotides to form a complementary strand.
• The DNA winds up into a double helix and overall, 2 complete DNA strands have formed.
• This is known as semi-conservative replication; as each DNA molecule now contains 1 'new' strand and 1 'old'strand.