B3 - Meosis and Mitosis Flashcards
Why do body cells divide?
• to replace worn out cells
• to repair damaged tissues
• to grow by producing more cells.
What is mitotis?
Body cells divide by mitosis. Each cell produces two genetically identical daughter cells. This increases the total number of cells in a multicellular organism.
Also used for asexual reproduction.
How does DNA replicate?
• DNA is a double-stranded molecule.
• The molecule ‘unzips’, forming two new strands.
• This exposes the DNA bases on each strand.
• Spare DNA bases in the nucleus line up against their complementary pair, forming base pairs.
• One molecule of DNA has become two identical
molecules.
How does mitosis happen ( in detail ) ?
- When each chromosome has made a copy of itself, these duplicated chromosomes line up across the centre of the cell.
• Then each ‘double’ chromosome splits into its two identical copies.
• Each copy moves to opposite ends of the cell.
• Two new nuclei form, each with a full set of
chromosomes.
• The cell divides into two genetically identical cells.
What are the advantages of being multicellular ?
- the organisms can be larger
- it can have different types of cells that do different jobs
- can be more complex
What is meiosis?
Cell division which makes gametes.
How does meiosis happen ( in detail )?
- Before the cell divides, copies of the genetic information are made, just as they are before mitosis.
• So each chromosome has an exact copy of itself.
• However, in meiosis, the cell divides twice, forming four gametes.
• In the first division the chromosomes pair up in their matched pairs.
• They line up along the centre of the cell.
• The members of each pair split up and go to opposite poles (ends) of the cells.
• These two new cells each divide again.
• This time the double chromosomes split and go to opposite poles.
• Four cells, each genetically different from each other and from the parent cell, and with only half the number of chromosomes of the parent cell.
What specialised organ systems do large multicellular organisms require?
• the nervous and endocrine systems, allowing communication between cells
• the circulatory system, supplying cells with nutrients
• the respiratory and digestive systems, controlling exchanges with the environment
Compare meiosis and mitosis
Mitosis :
- produce two diploid cells
- both genetically identical to themselves and the parents
- goes through one division
- used to repair cells, growth, and asexual reproduction
Meiosis :
- produce four haploid cells
- each genetically different from themselves and the parents
- goes through two divisions
- chromosomes cross over / swap for variation
- only used to create gametes
What is a zygote? How do they form?
When two haploid gametes (an egg and a sperm) join, they produce a diploid cell called a zygote. This zygote will divide by mitosis into many cells and grow into a new individual.
How does fertilisation cause variation?
• The joining of two gametes is called fertilisation.
• The combining of genetic material from two parents produces a unique individual.
• Half its chromosomes (and genes/alleles) have come from one parent and half from the other parent.
• It will have two sets of chromosomes.
• The combination of alleles will control the characteristics
of the individual resulting from the zygote.
How are sperm adapted to their function?
They :
• are small and have a tail so they can swim to the egg cell
• have a nucleus to carry their genetic material
• are made in large numbers to increase the chance that one will find the egg
• have many mitochondria to provide a lot of energy
• have an acrosome that releases enzymes to digest (break down) the egg membrane.
