| what is external fertilisation | it occurs in a medium such as water – both eggs and sperm are released into water |
| what is one disadvantage of external fertilisation | survival rates of eggs are low as not all become fertilised – females produce and release lots of eggs |
| what is an advantage of external fertilisation | allows there be a greater genetic variation increasing the chance of survival |
| what is internal fertilisation | mainly occurs in land-based animals – fertilisation occurs within the female |
| what are three main types of internal fertilisation | ovoviparity, oviparity, and viviparity |
| what is oviparity | - type of internal reproduction
fertilised eggs are laid out of the female’s body and are nourished by the yolk which is contained inside the egg |
| what is ovoviparity | - type of internal reproduction
fertilised eggs remain in female however the embryo gains nourishment from the egg yolk – young are fully developed when birthed – occurs in some sharks, lizards, and snakes |
| what is viviparity | - it is a form of internal reproduction
embryo develops in the female and gain nutrients from the mother’s blood through the placenta – offspring is birthed alive |
| what is one advantage and one disadvantage of reproducing internally | disadvantage - fewer offspring are produced in this method
- advantage - survival rates are higher as there is a higher chance the egg is fertilised and there is less predation |
| list three advantages of sexual reproduction | - long-term evolutionary potential
- deleterious genetic variations are exterminated quickly
- generates genetic variation
- populations can better adapt to changing environments |
| list three disadvantages of sexual reproduction | - slower reproductive rate
- recombination can break down beneficial genomic combinations
- deleterious variations can occur
- potential for sexually transmitted diseases to be spread
- it is costly in terms of energy |
| what is meiosis | - Produces haploid (n) cells so that when they combine in fertilisation the zygote is diploid (2n)
- It’s a reduction division as it reduces the amount of chromosomes to half of what somatic cells have |
| what is a reduction division | it is the first cell division in meiosis, the process by which germ cells are formed
- In reduction division, the chromosome number is reduced from diploid (46 chromosomes) to diploid (23 chromosomes) |
| how many sperm are created in meiosis | 4 |
| how many eggs are created in meiosis | only 1 egg due to uneven split of cytoplasm – the left-over cells are called polar bodies |
| what are polar bodies | they are cells produced by meiosis that are not large enough to be eggs so die via apoptosis |
| what are the two phases of meiosis | meiosis I and meiosis II |
| what are the 8 sub-phases of meiosis | meiosis I - prophase I, metaphase I, anaphase I, as well as telophase I and cytokinesis
meiosis II - prophase II, metaphase II, anaphase II, as well as telophase II and cytokinesis |
| what happens in prophase I | chromosomes condense, and crossing over of homologous chromosomes occurs in late prophase |
| what happens in metaphase I | crossing over occurs in early metaphase, each chromosome is attached to a spindle fibre and homologous chromosomes line up along the equator |
| what happens in anaphase I | spindle fibres draw chromosomes to opposite poles of the cell, and cell membrane begins to pinch |
| what happens in telophase I and cytokinesis | cytoplasm divides, nuclear membranes form – two daughter diploid cells are produced |
| what happens in prophase II | nuclear envelope breaks down again and meiotic spindle reforms |
| what happens in metaphase II | chromosomes are connected to a spindle fibre and line up along to the equator |
| what happens in anaphase II | spindle fibres draw chromosomes to opposite poles of the cell, and cell membrane begins to pinch |
| what happens in telophase II and cytokinesis | cells split in to two daughter cells – 4 cells in total – nuclear membranes reform |
| what is synapsis | process that occurs in the first division of meiosis
– matching homologous chromosomes pair up along the equator of the cell |
| what is crossing over and when does it occur | process where chromosomes swap genetic material
→ occurs during late prophase and early metaphase
→ occurs at the chiasma – molecular scaffold is temporarily created which is later disassembled |
| what is the chiasma | it is where crossing over occurs
- it is a molecular scaffold that is temporarily created and later disassembled |
| what is recombination | the process of producing chromosomes with new genetic combinations |
| where does meiosis occur | occurs in specialised organs called gonads (testes and ovaries) |
| what are gonads | they are sexual reproduction glands - ovaries in women and testes in men |
| why is meiosis important | - it ensures all offspring have the correct number of chromosomes
- it also ensures genetic variation between parent and offspring through recombination – improving survival traits when certain factors change |
| what are the three main differences between meiosis and mitosis | - Mitosis doesn’t have recombination, so all daughter cells (2) are genetically identical whereas in meiosis daughter cells (4) are all genetically unique
- There are two daughter cells after mitosis which are diploid and contain 23 chromosomes wherever after meiosis there are four haploid daughter cells with 23 chromosomes
- Homologous pairs synapse in meiosis and not in mitosis |
| what is spermiogenesis | Spermiogenesis is the process by which 4 sperm are created from spermatids |
| what are spermatids | they are cells that undergo meiosis to transform in to 4 sperm |
| what are some characteristics of sperm | they contain limited food reserves and are mobile |
| what is oogenesis | Oogenesis is the process by which 1 egg and 3 polar bodies are formed from oogonial stem cells |
| what are oogonial stem cells | they are cells that undergo meiosis to form one egg and three polar bodies |
