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
Describe the process of M1 P2
- Chromsomes condense and become visible again, and the nuclear membrane will break down
-Splindle fibres then begin to develop
Describe the process of M1 M2
- Chromsomes line up on the equator of the splindile apparatus
Describe the process of M1 A2
- Centromere of each chromsome divides
- Splindle fibres shorten
- Chromatids are now pulled to opposite poles of the cell
Describe the process of M1 T2
- Chromatids have reached the poles of the cell
- Nuclear membranes reform
-Chromosomes uncoil back to their chromatid state
Each cell then undergoes cytokenesis forming 2 haploid cells
What is the equation for the number of genetically different gametes formed by independent segregation
2 to the power of homologous chromsome pairs
Describe the function of the centromere during mitosis 2 marks
Holds chromatids together#
Attaches chromatids to splindle
So allows chromatids to seperate
Why are homologous chromosomes not genetically identical?
They carry the same genes but different alleles
Describe the process of binary fission
Circular dna and plasmids replicate
Cytoplasm then divides into two to create two daughter cells
Why dont viruses undergo cell division?
They are non living and acellular
This means they only reproduce by infecting living cells known as host cells
How do viruses replicate
They inject their nucleic acid into host cells
Virus uses attachement proteins in order to bind to complementary receptor proteins on the surface of the host cell
The virus then injects its DNA and RNA in the host cell
The host cell then uses its nucleic acid and riobosomes to build new viruses
What is the mitotic index equation
Number of cells in mitosis divided by number of cells in field of view
Describe the chromosome behaviour in prophase and metaphase of mitosis
In prophase chromosomes condense and become visible
IN metaphase they line up at the equator of the cell and are attached to splindle fibres via the centromere
Describe differences between meosis and mitosis
Mitsosis there is one division producing two daughter cells however in meosis there are two divisons causing there to be 4 daughter hiploid cells
Mitosis provides genetically identical cells whereas meosis provides genetically different
In meosis crossing over occours in mitosis it doesnt
IN meosis homologous chromosomes assosiate whereas in mitosis they dont
Describe how the process of meosis results in 4 haploid cells
DNA is replicated
There are 2 rounds of cell division
During first round homolougous chromsomes are seperated
During second round sister chromatids are seperated to different poles of the cell
Describe binary fission of bacteria
Circular dna replicates
Plasmids replicate
Cytoplasm divides to form two daughter cells
Use image to describe what is happening in the image
Chromomes are condensing
Chromosomes become visible
Describe the behaviours of chromosomes and roles of splindle fibres during P, M , A
Prophase- Chromosomes condense and become visible, splindle fibres attach to the centromeres of each chromosomes
Metaphase- Chromosomes line up at the equator of the cell , centromeres attach to splindle fibres
Anaphase- Splindle fibres shorten and pull individual chromatids they are bound to to opposite poles of the cell
What might cause an offspring to be sterile?
Too many copies of chromosomes
Homologous chromosomes do not pair
Homologous chromsomes do not separate evenly
So no meosis
Why would the stem of the onion root be used
This is where dividing cells are found, mitosis is happening
Pressed down on cover slip in experiment
To make a single layer of cells so light can pass through
Explain what to do to make a mitotic index count more accurate
To ensure a representive sample
Examine a large field of view
Give aseptic techniques used to observe a sample of bacteria
Wear gloves or a mask to prevent contamination from hands
Flame the loop to maintain a pure culture of bacteria
Open lid of petri dish as little as possible to prevent unwanted bacteria contaminating the dish
Apart from crossing over, independent seg and mutations how is genetic variation inc?
Random fusion of gametes / fertalisation
produces new allele combinations
Difference between addition mutation and substitution mutation
Addition results in a frameshift, all triplets are moved, would affect more than one amino acid in the sequence
Subsitution would mean an amino acid is replaced
