| what is non-disjunction? (2) | when chromosomes don't separate properly = extra or missing chromosomes
aneuploidy |
| what is meiosis? | how sex (haploid cells) are made = germline cell = 4 cells |
| how many divisions are there in meiosis? | 2 |
| what happens during the two divisions to create 4 haploid cells? (2) | 1) separates homologous chromosomes to half the chromosome number (haploid)
2) separates sister chromatids (made from DNA replication in interphase |
| when is DNA replicated and in which specific phase? | interphase - s phase |
| what does DNA replication in interphase form? | 2 genetically identical copies = sister chromatids (1 chromosome) |
| what are the phases of meiosis? (PMAT) | prophase
metaphase
anaphase
telephase |
| What is the second growth phase between meiosis I and meiosis II called? | interkinesis |
| Describe the different stages in meiosis I? (4) | P = chromosomes condense, membrane dissolves and crossing over happens with paired chromosomes (bivalents)
M= spindle fibres connect to bivalents to align them in the middle
A = spindle fibres contract and homologous chromosomes are pulled away to opposite ends
T = chromosomes decondense and cell divides to 2 haploid cells through cytokinesis |
| Describe the stages of meiosis II? | P = chromosomes condense, membrane dissolves
M = spindle fibres attach to chromosomes and aligns them in the middle
A = spindle fibres pulls sister chromatids away seperating them
T = chromosomes decondense, membrane reforms and cell divides to form 4 haploid cells |
| Why can the 4 haploid cells from meiosis be genetically distinct? | because of crossing over in prophase 1 can cause recombination |
| what is the process called whereby chromosomes forms pairs? what are these pairs called? | synapsis forms bivalents |
| how are the pairs of chromosomes held together in prophase 1? | by the chiasmata |
| how and where does crossing over occur? | in prophase 1 in meiosis crossing over occurs at the chiasmata which forms recombinant chromatids by exchanging genetic material |
| how does metaphase 1 lead to more variation? | Due to random independent assortment of chromosomes into the gametes when aligning in the middle |
| what would happen if the chromosome numbers were not halved? | the total chromosome numbers would double in each generation = polyploidy |
| what are the 3 ways genetic variation is achieved? | crossing over in prophase 1
random assortment in metaphase 1
random fusion of gametes |
| what is non-disjunction? (2) | when chromosomes don't separate properly = extra or missing chromosomes
aneuploidy |
| when can non-disjunction happen? (2) | when chromosomes don't separate in anaphase 1 = affects all 4
when chromosomes dont separate in anaphase 2 = affects 2 |
| how is down syndrome caused? | 3 copies of chromosome 21 ( trisomy) |
| what can influence the chances of non-disjunction occuring? | maternal age because of developing oocytes being in p1 until ovulation
especially after 30
mostly from non disjunction in meiosis 1 |
| what is karyotyping? how is it done? | chromosomes are organised to find genetic abnormalities
cells are harvested from fetus before cell division so that the chromosomes are visible |
| what is chorionic villi sampling? | removes placental tissue at 11 weeks to find CS abnormalities
1% risk of miscarriage |
| what is amniocentesis | amniotic fluid at 16 weeks to find CS abnormalities with 0.5% chance of miscarriage |
| what are the differences between mitosis and meiosis? (8) | Division – Mit 1 division mei 2 divisions
Independent assortment – Homologous pairs are randomly separated into separate cells only in meiosis
Synapsis – Homologous pairs form bivalents in meiosis only
Crossing over – Non-sister chromatids of homologous pairs may exchange genetic material in meiosis only
Outcome – Mitosis forms 2 cells meiosis forms 4
Ploidy – mitosis forms diploid while meiosis forms haploid
Use – Mitosis is used to clone body cells, while meiosis is used to generate sex cells (gametes)
Genetics – Cells produced by mitosis are genetically identical (clones), while cells produced by meiosis are genetically distinct |
