Unit 3- Eukaryotic Cell Division Flashcards
What is chromatin?
- coiled DNA wrapped around a histone protein
How is chromatin different to chromosomes
Chromatin are bundles of histone proteins wrapped in DNA
Chromosomes are millions of coiled up chromatins
Where are chromatids joined?
At the centromere
Why can we see chromosomes in a light microscope?
They are short very wide bundles of chromatin that are so thick and coiled we can see them
What is the product of mitosis?
2 genetically identical daughter cells
What is mitosis used for?
- Growth
- replacement
- repair
- asexual reproduction in some eukaryotic organisms
- artificial cloning of plants and animals
What are the 3 phases of the cell cycle?
Mitosis
Cytokinesis
Interphase
What phase do cells spend the majority of their time in?
Interphase
Describe interphase:
Growth 1 phase: cell grows to original size, organelles are replicated and genes are expressed
Synthesis phase: DNA and histone are replicated
Growth 2 phase: spindle and proteins are synthesised
Summary of mitosis phase:
Nucleus divides to make 2 nuclei with identical copies of DNA
This is subdivided into PMAT
Summary of cytokinesis:
The division of the cytoplasm to form 2daughter cells, with 1 nucleus each
Prophase: (3)
- Chromosomes condense becoming visible
- centrioles move to opposite poles of cell
- nucleoli disappears
Metaphase:
- nuclear envelope disappears
- chromosomes align along equator of cell
- spindle fibres connect centrioles to chromosomes
Anaphase: (3)
- centromere splits, chromatids separate
- spindle fibres shorten pulling chromatids towards poles, centromere first
- some spindle fibres lengthen pushing the poles further apart
Telophase: (3)
- spindle fibres disperse
- nuclear envelope forms around 2 sets of chromosomes
- chromatids (chromosomes) uncoil becoming too thin to see
Cytokinesis, how it differs in plants and animals:
Animals: ring of actin filament forms around cell equator and tightens squeezing the cell into cleavage furrow which splits into 2 cells
Plants: vesicles move to cell equator, line up and fuse a cell plate down the middle which developers into a cell wall
What are the products of meiosis?
4 genetically different haploid daughter cells
Describe the differences in division 1 of meiosis:
Prophase: homologous chromosomes join together to for bivalents
Metaphase: bivalents line up on equator of cell
Anaphase: bivalents are separated and homologous chromosomes are pulled to opposite poles of the cell (chromatids don’t separate)
Telophase: chromosomes don’t uncoil
What are bivalents and where are they found?
Meiosis division 1:
2 homologous chromosomes joined together
How does meiosis differ from mitosis?
In meiosis the chromosome number is halved
In meiosis chromosomes are rearranged to form new combinations of alleles, this os genetic recombination
Why are does meiosis produce gametes with a haploid number of chromosomes?
As fertilisation later restores the haploid number to diploid
What are the 2 processes that occurs in meiosis that result in genetic variation?
Crossing over
Independent assortment
When does crossing over occur?
Prophase 1 of meiosis, when homologous chromosomes bind to form bivalents
What is crossing over?
When homologous chromosomes join together to form bivalents, sections of sister chromatids cross over, resulting in the swapping of alleles
How come chromosomes stay the same length in crossing over?
Equal amounts of the chromosomes cross over
When does independent assortment occurs?
Metaphase 1 of meiosis
What is independent assortment?
When the random line up of bivalents along the cell equator gives rise to genetic variation by recombination of alleles
What are the 2 different types of chromosome mutations?
Nondisjunction and translocation
What is non-disjunction?
This is when chromatids or chromosomes remain stuck together in anaphase 1 or 2, moving together to one pole of the cell
What is the product of nondisjunction? And what if the gamete fuses with the other gamete?
Gametes with extra chromosomes or missing chromosomes, if this gamete fertilises the zygote shows polysomy
What is Down syndromes link to non-disjunction?
- when a gamete with an extra chromosome fertilises a normal gamete the zygote will show trisomy
- 3 copies of the smallest chromosome 21
Symptoms of Down’s syndrome
- recognisable facial features
- short height
- heart defects
- poor vision
- learning difficulties
What is Turner syndrome caused by?
When a gamete with a missing X chromosome fertilises a normal gamete
The zygote shows monosomy
Symptoms of Turner syndrome?
Female sex organs don’t mature at adolescence
What is monosomy and trisomy?
- having only 1 copy of a chromosome
- having 3 copies of a chromosome
What is polyploidy?
When all the chromosomes remain stuck together in meiosis, forming a gamete with 2 copies of every single chromosome which fertilises a normal haploid gamete resulting in a Triploid gamete
What is translocation mutation?
When a part of a chromosome is swapped with a part of a completely different non-homologous chromosome.
When do translocation mutations occur?
During interphase
What causes translocation?
Radiation, virus’ or chemicals which spontaneously break the DNA molecules and the DNA ligase joins the DNA back in the wrong place
Why are translocation mutations most often harmless?
Exactly the same genes and alleles are still present, just located on different chromosomes
Cells with translocation mutations have difficulties with meiosis, why is this?
Homologous chromosomes are no longer homologous (eg, paternal chromosomes may have swapped over sections)
True bivalents cannot form, so the gametes produced from these bivalents are unviable and cause miscarriages
