6.1 -6.3- The Cell Cycle, Mitosis, Meoisis Flashcards
What is the cell cycle?
The process that all body cells in multicellular organisms use to grow and divide
How does the cell cycle start initially?
Starts with a cell that has already been produced by cell division and ends with this cell dividing to produce 2 genetically identical daughter cells
What is mitosis?
The division into two daughter cells that are genetically identical to each other and to the parent cell
What is meiosis?
• Division into four unique daughter cells with half the chromosomes of the parent cell
• a form of cell division that produces 4 genetically different haploid cells ( cells with half the number of chromosomes found in the parent cell ) known as gametes
• process by which sex cells are made in the reproductive organs , it involves the reproduction division of a diploid germ line cell into four genetically distinct haploid nuclei
What are the stages of the cell cycle?
- Interphase
- Mitosis or meiosis ( nuclear division)
- Cytokinesis ( cytoplasmic division)
What is a haploid?
• One copy of each chromosome
• three non homologous chromosomes
What is a diploid cell?
• 2 copies of each chromosome
• three pairs of homologous chromosomes
Why is the cell cycle important?
• produces genetically identical daughter cells
• growth of tissue / organism
• replacement of worn out / damaged cells
• repair of body tissues ( eg. Bone , muscle etc. )
• asexual reproduction/ cloning
What occurs in the interphase?
- G1 ( growth phase 1)
- S ( DNA synthesis)
- G2 phases ( growth phase 2 )
What occurs in the G1 interphase stages ?
First growth phase of cell, synthesis of proteins, organelles replicate eg. Mitochondria, ribosomes etc. Cell increases in size
What occurs in the S, interphase stages?
• replication of each chromosome in the nucleus
• they’re called sister chromatids ( joined at the centromere )
What occurs in the G2 stage in the interphase stage ?
• cell continues to grow in size
• duplicated DNA is checked for errors
• energy stores are increased
What are the cell cycle check points?
- M phase
- Gap phase 1
- G1 check points
- Synthesis
- Gap phase 2
- G2 checkpoint
What is the purpose of mitosis?
Produces 2 genetically identical daughter cells for:
• growth
• cell replacement/ tissue repair
• asexual reproduction
Name the stages of mitosis
1.Prophase
2. Metaphase
3. Anaphase
4. Telophase
What happens in interphase?
• each DNA molecule in the nucleus is replicated during the interphase
• each chromosome is converted into two identical DNA molecules called Chromatids
• the two chromatids are joined together at a region called the centromere
• condensed chromosomes combined with Justine proteins are called chromatids
What occurs in prophase?
- Chromosomes condense and thicken ( thus become visible )
- Consists of sister chromatids (2) joined at the centromere
- Two centrioles migrate to opposite poles of the cell
- Spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre equator of the cell
- Nucleur envelope disappears
What occurs in metaphase?
- Brief phase
- Individual sister chromatids are moved by the spindle fibres to align at the metaphase plate/ equator at the centre of the cell
- Sister chromatids are attached to the spindle fibres by the centromere
What occurs in anaphase?
- Centromeres holding the pairs of chromatids in each chromosome divide
- Sister chromatids separate
- Spindle contracts - fibres shorten
- Each chromatid is pulled by their centromere to opposite poles of the cell
What occurs in telophase?
- Chromatids have reached opposite poles of the cell
- They uncoil and become long and thin again
- They are now called chromosomes
- Spindle fibres disappear
- Nuclear envelope reforms and enclose around the chromosomes at each pole
What is the G^0 phase ( resting state ) ?
Phase where the cell leaves the cell cycle either temporarily or permanently
What may happen if DNA is not checked?
• mutations in the DNA sequence
• faulty DNA produced
• error in copying daughter cells
• proteins not made or do not function properly
What are homologus chromosomes?
• Pair of chromosomes with genes at the same locus
• 1 maternal and 1 paternal
• some alleles maybe the same while others are different
What are sister chromatids?
Identical, carry the same versions of all their genes because one was produced as an exact copy of the other
What is the chromosome structure?
• only visible during cell division
• consists of two chromatids joined somewhere along its length at the centromere
• genetic information ( genes / alleles ) carried on each chromatid is identical
What occurs in cytokinesis?
This is the phase where the cell surface membrane and cytoplasm divide
What occurs in animal cells in cytokinesis?
A clearage furrow forms
What occurs in plant cells in cytokinesis?
A cell plate forms
How is the cell cycle regulated?
• checkpoints regulated by cell signalling proteins ensure damaged cells do not progress to next stage of the cycle
• cyclin- dependent kinase enzymes phosphorylate proteins that initiate next phase of reactions
Describe what happens at each key checkpoint in the cell cycle
• Between G1 & S - cell checks for DNA damage- after restriction point - cell enters cycle
• Between G2 & M - cell checks chromosomes replication
• At metaphase checkpoint - cell checks that sister chromatids have attached to spindle correctly
What happens during meiosis I ?
- Homologous chromosome pair to form bivalents
- Crossing over occurs at chiasmata
- Cell divides into two
- Homologous chromosomes separate randomly
- Each cell contains either maternal or paternal copy
What happens during meiosis ii?
- Independent segregation of sister chromatids
- Each cell divides again producing 4 haploid cells
What is the importance of meiosis?
• takes place in sex organs
• gametes produced here
• important to have genetically different gametes
• promotes genetic variation and allows for natural selection to take place
How does meiosis produce genetic variation?
• During meiosis I homologous pairs of chromosomes swap parts of their genetic material- this is crossing over
• the chromosomes from each pair are randomly allotted to the daughter cells by independent assortment
What occurs in prophase 1 in meiosis ?
• chromosomes condense
• nuclear envelope dissolves
• spindle formation begins
• homologous chromosomes pair up forming bivalents
• crossing over ph genetic information occurs
Explain what happens during crossing over in prophase 1
• homologous chromosomes undergo a process called synapsis - where by they pair up to form a bivalent
• the homologous chromosomes are held together at points called chiasmata
• crossing over of genetic material between non-sister chromatids can occur at these chiasmata
• as a result of this exchange of genetic material = new gene combinations are formed on chromatids (recombination)
• once chiasmata are formed - homologous chromosomes condense as bivalents and then are separated in meiosis
• if crossing over occurs then all 4 haploid daughter cells will be genetically distinct- sister chromatids are no longer identical
What occurs in metaphase 1 in meiosis?
• spindle fibres from opposing centromeres connect to bivalents at centromeres and align them along the middle of the cell
• Homologous pairs of chromosomes assemble along the metaphase plate
• the orientation of each homologous pair is random and independent of any other homologous pair
• this is called independent assortment
What occurs in anaphase 1 in meiosis?
- Spindle fibres contract and split the bivalent , homologous chromosomes separate
- Each homologous chromosome moves to opposite sides of the cell
- The result is that 23 chromosomes move to one pole - 23 chromosomes move to the other pole
How does meiosis introduce variation into the haploid daughter cells?
• independent assortment of chromosomes during anaphase 1
• when homologous pairs line up along the equator of the cell during metaphase 1 and get separated during anaphase 1 of meiosis 1 it is completely random which chromosome from each pair ends up in the daughter cell
• this gives rise to new combinations of alleles
What occurs in telophase 1 in meiosis?
• chromosomes assemble at each pole and Decondense
• nucleur envelope may reform
• cell undergoes cytokinesis to form 2 haploid daughter cells
What occurs in prophase 2 in meiosis?
- Chromosomes pair up and re- condense
- Nuclear envelope breaks down again
- Spindle fibres reform
- Centrioles move to opposite poles of the cell
What occurs in metaphase 2 in meiosis?
Chromosomes are lined on the equator by the spindle fibres
What occurs in anaphase 2 in meiosis ?
- Centromere divides
- Sister chromatids are separated
- Spindle fibres contract
- Chromatids ( now called chromosomes) move to the poles of the cells
What occurs in the telophase 2 in meiosis?
- Chromatids uncoil and decondense
- Spindle fibres break down
- Nuclear envelope reform
- Cell undergoes cytokinesis
- 4 haploid daughter cells are produced
What occurs in cytokinesis in meiosis?
Cytoplasm and surface membrane divide, creating four independent haploid daughter cells
