Cell division Flashcards
Cell division by mitosis and meiosis, binary fission
What does mitosis produce?
- Produces two daughter cells that have the same number of chromosomes as the parent cells and eachother
Describe what occurs during the interphase period of mitosis
This period precedes mitosis, and this is where the cell is not dividing
- The organelles double
- The cells grow
- DNA replicates
- Energy stores increase
- The two copies of DNA that have been replicated stay at the centrimetre
What is a diploid cell?
A cell that has its chromosomes in pairs
What occurs in the mitotic phase of the cell cycle?
-Nucleus divides during mitosis into two daughter nuclei
- Cell divides during cytokenesis into two daughter cells
When can we and can we not see the chromosomes during the cell cycle?
The mitotic phase, mitosis and cytokenesis we can see chromosomes
In inter-phase we can only see chromatin, a dark material where chromosomes have a loose structure so DNA is accessible for transcription and translation
How are chromosomes replicated during interphase of the cell cycle?
Chromosomes will split into two individual sister chromatids which are joined at a centromere, this happens for all chromsomes
Chromsomes are not visible at this stage
Describe the details of prophase in mitosis
- Chomosomes condense and become visible in the nucleus
- Each chromsome consists of two sister chromatids joined at the centromere
- The nucleolus dissapears
- Nuclear envelope breaks down
-A pair of centrioles move to opposite poles of the cell - From each centriole, splindle fibres develop which span from pole to pole as they attach to the centromere of each chromsome
-Chromsomes are drawn to the equator of the cell by splindle fibres attached to the centromere
Describe the details of what happens during metaphase in mitosis
At metaphase the splindle apparatus has fully formed
- Chromosomes are lined up at the equator of the cell
Describe the details of what happens during anaphase of mitosis
- The centromeres divide into two
- This is because the splindle fibres start to retract and pull the centromere and chromatids they are bound to towards opposite poles
- This stage relies on energy in the form of ATP provided by respiration in the mitochondria
Describe the details of what happens during telophase of mitosis
- Chromatids have reached the opposite poles of the cell and become longer and thinner, these are now referred to as chromosomes again
- Splindle fibres disentegrate and the splindle apparatus breaks down
- Nuclear envelope and the nucleolus then reform
- At this point , the cell has undergone nuclear division by mitosis
What happens during the cytokenesis stage of mitosis
- The central cell membrane is pulled inwards by the cytoskeleton
- This creates a groove which deepens
-The membrane then fuses to form two individual cells
How is cell divison by mitosis different in plant cells to in animal cells?
In plant cells, centrioles do not play a role in forming the splindle apparatus in plant cells as they dont contain centrioles
- During cytokenesis, the cell membrane cant form a groove like in animal cells
- Instead, vesicles from the golgi apparatus form membrane structures down the centre of the cell, these then fuse together to form the central cell membrane dividing the cytoplasm into two
- A cellulose cell wall now forms in the middle of the cell
What type of cells does meosis form?
Gametes e,g egg sperm in animals and pollen and egg in plants
What types of cells does meosis start and end with?
Begins with a singular diploid cell and finishes with 4 haploid daughter gametes
Why is it important that gametes are haploid cells?
Because gametes fuse together during fertalisation to produce a fertalised egg or a zygote, as gametes are haploid, the zygote has the diploid number of chromosomes
What is important about the gamete cells produced during meosis?
They are all genetically different
What is a homologous pair?
Chromosomes in a pair with exactly the same genes
Carry these genes in the same loci
The alleles of these genes can be different for each chromsome in a homologous pair
Describe the process of crossing over in homologous chromosomes
- Before the nucleus divides by meiosis all chromosomes are copied during interphase, these sister chromsomes are joined at the centromere
- In prophase, the chromatids of the two chromosomes wrap around eachother and form bivalents in a process called crossing over
- The points where these chromosomes cross over are called chiasmata
- Then parts of these chromatids at chiasmata can break off and exchange between the homologous chromosomes meaning the maternal and paternal chromsomes have now exchanged DNA
- These are recombinant chromsomes, as these chromsomes have exchanged DNA this means they can exchange alleles, meaning a large combination of genotypes can be formed
Why might crossing over of homologous chromosomes cause each haploid gamete to be genetically different?
-Chiasmata can occur in multiple different positions on each sister chromatid
A large number of alleles can then be exchanged
Describe the process of independent segregation of homologous chromsomes
-During M1 M1 homologous chromsomes line up opposite eachother on either side of the equator
- There are 2 to the power of 23 different ways that the pairs can assort themselves because it is random each time which side of the equator that the maternal and paternal chromosome of each homologous pair align at the equator, meaning each gamete recieves different combinations of the maternal and paternal chromsomes
- The orientation of each homologous pair is random and is not affected by the orientation of any other homologous pair
This means an allele on one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome (their inheritance is independent of one another)
Describe the process of M1 P1
- The cell has already gone through interphase, where the cell has copied its chromsomes and its organelles, chromsomes are not visible as distinct structures during this phase
- First stage is prophase 1, where chromsomes condense and become visible, and homologous chromsomes will wrap around eachother forming a bivalent, and where these homologous chromosomes link together chiasmata are formed, where crossing over takes place and alleles are exchanged between homologous chromsome.
During P1, nuclear membrane breaks down and centrioles move to opposite poles of the cell and a splindle appartus forms
Describe the process of M1 M1
The pairs of homologous chromsomes are lined up on the equator of the splindle appartatus
Describe the process of M1 A1
- Splindle fibres now shorten and homologous chromsomes move to opposite poles of the cell
- This means chiasmata on homologous chromsomes break
Describe the process of M1 T1
-Chromsomes have reached the opposite poles of the cell
- Nuclear membranes reform
-Chromosomes uncoil back into their chromatin state
Cytokenesis will now take place dividing into two cells and these cells are now haploid cells as they no longer contain homologous pairs of chromosomes
