4.3- Origin Flashcards
- What is the definition of a haploid cell? How many chromosomes does a human haploid cell? Give an example of a haploid cell? Draw an example
• Haploid cells contain just one copy of each chromosome.
• In humans haploid cells have 23 chromosomes.
The gametes are examples (egg and sperm cel
- Why is it so important that gametes are haploid?
TO MAINTAIN CHROMOSOME NUMBER AT FERTILISATION
- What is a the definition of a diploid cell? A human diploid cell has how many chromosomes? Draw an example
- Compared with diploid cells which contain 2 copies of each chromosome.
- In humans there are 46 chromosomes (23 pairs) in a diploid cell
- What is an allele?
different forms of the gene
- What are homologous chromosomes and explain why they are not genetically identical?
- Homologous chromosomes have the same genes at the same loci
- They are not genetically identical as they different forms of the gene (alleles).
- At what division does crossing over occur? What is crossing over? Why does crossing over occur?
- Crossing over happens during the first division of meiosis.
- Crossing over is when the ‘arms’ of homologous pairs twisted up.
- The point at which the cross over is called a chiasma
- This leads to the exchange of alleles between homologous chromosomes
- THIS IS A RARE PROCESS
- At what division does independent assortment occur? What is it? And why does it occur?
• Independent assortment happens during the first division of meiosis.
• It is when the homologous pairs separate and go to opposite poles of the cell.
Independent assortment means that the combination of chromosomes that go into each gamete is random.
- Give 2 reasons that it is vital for meiosis to take place
- To make gametes which are haploid – so that when fertilisation takes place the chromosome number is maintained from one generation to the next
- To create genetic variation in gametes and offspring
- Describe what happens to chromosomes in meiosis.
- DNA replication takes place so that each chromosomes replicates
- The chromosomes become condensed
- Homologous chromosomes pair up (forming bivalents)
- Crossing-over can occur where there is an exchange of alleles between homologous pairs
- spindle fibres attach to the chromosomes at their centromere
- so the bivalents line up at the equator of the cell
- The spindle fibres contract causing the homologous chromosomes to separate and be pulled to opposite poles of the cell
- In the 2nd division of meiosis the sister chromatids are separated
- HOW does meiosis cause variation?
Crossing-over
- Independent segregation of homologous chromosomes in meiosis I (1st division)
- Independent segregation of chromatids in meiosis II (2nd division)
- Explain the advantage of variation?
Any
- Different adaptations / some better adapted;
- Some survive
- To reproduce;
- Pass on gene / allele;
- Allows for survival in changing environment / different environment
Mitosis
Maintains the same chromosome number as in the parent nucleus Homologous chromosomes do not pair up No crossing-over One division / → 2 offspring cells Genetically identical cells produced
Meiosis
Reduces the chromosome number Homologous chromosomes associate in pairs Crossing-over / chiasmata formation Two divisions / → 4 offspring cells Genetically different cells produced
Meiosis
Original diploid cell After DNA replication After the first meiotic division After the second meiotic division
Your drawing
e.g.
Description
The cell starts
DNA replication happens, so each chromosome is made of 2 identical sister chromatids held together by a centromere.
The homologous pairs then line up next to each other along the equator of the cell The homologous pairs are separated by the spindle fibres which pull them to opposite poles of the original cell.
Each daughter cell that forms is haploid.
(NOTE: crossing over and independent segregation occur during the first meiotic division) The sister chromatids are separated by the spindle fibres which pull them to opposite poles of the original cell.
- Number of chromosomes = 4
- Number of homologous pairs = 2
- Amount of DNA = 60 (in arbitrary units) • Number of chromosomes = 4
- Number of homologous pairs = 2
- Amount of DNA = 120 (in arbitrary units) • Number of chromosomes = 2
- Number of homologous pairs = 0
- Amount of DNA = 60 (in arbitrary units) • Number of chromosomes = 2
- Number of homologous pairs = 0
- Amount of DNA = 30 (in arbitrary units)
DIPLOID
Diploid Haploid haploid
