Topic 4-Meiosis Flashcards
What is a diploid cell?
-A cell containing two of each chromosome
What is the diploid number?
2n
What are alleles?
Chromosomes that make up each pair are the same size and have the same genes although they could have different versions of those genes.
What are homologous chromosomes?
Pairs of matching chromosomes that are the same size but could have different alleles
How many homologous pairs does a human have?
23 pairs
What is the diploid number for humans?
46
What are gametes?
The sperm cells in males and egg cells in females
Do gametes have a haploid number of cells or diploid?
Haploid
What is a zygote?
-In sexual reproduction two gametes join togther in fertilisation
-It then divides and devolps into a new organism
What happens during fertilisation?
-A haploid sperm fuses with a haploid egg
-This makes a cell with the normal diploid number of chromosomes
-Half of these chromosomes are from the father and are from the mother
Is fertilisation random in sexual reproduction?
-Yes
-Any sperm cell can fertilise an egg
What does random fertilisation produce?
Zygotes with different combinations of chromosomes from both parents
What is the effect of mixing genetic material in sexual reproduction?
-Increases genetic diversity within a species
Where does meiosis take place?
-In the reproductive organs of multicellular eukaryotic organisms
Cells that divide during meiosis are diploid, to begin with, but what are the cells that are formed from meiosis? Why?
-Haploid
-The chromosome number halves
Does meiosis in humans and mammals produce gametes directly?
Yes
In other organisms are gametes produced directly?
-No
-It produces haploid cells which later divide by mitosis to become gametes
Without meiosis what would happen when the gametes fused?
-You would get double the number of chromosomes
Step 1: What happens before meiosis starts?
-The DNA unravels and replicates so there are two copies of each chromosome called chromatids
(Organelles and centrioles replicate)
What happens during interphase 1?
-The cell grows, organelles are produced and the cell carries out its normal functions
How is DNA arranged in interphase?
-Long, diffuse (spread-out) and chromosomes are not visible
What happens during prophase 1?
-The chromosomes condense and become fatter
-The chromosomes appear as a double structure consisting of 2 sister chromatids joined by a centromere
-Centrioles begin to migrate towards the poles
-The spindle fibres begin to form extending from the centrioles
-The nuclear membrane and nucleolus breaks down in late prophase/ early metaphase
-Homologous chromosomes come together to form a bivalent (synapsis)
What happens during metaphase 1?
-Centrioles reach the poles
-Microtubules extend between centrioles to form spindle fibres
-Bivalents line up on the equator and attach to the the centromere
-One homologous chromosome is attached by microtubules to one pole and the other is attached to the opposite pole
What is the product of meiosis?
-Four haploid cells that are genetically different from each other
What are the two ways genetic variation is created in gametes?
-Crossing over of chromatids
-Independent segregation
When does crossing over occur?
-During prophase one
What happens during crossing over?
-The chromosomes in each homologous chromosome pair twist around each other
-Chromatids break and rejoin
-The chromatids still contain the same genes but now have a different combination of alleles
What is the result of crossing over?
-The four daughter cells formed from meiosis 2 contain chromatids with different alleles
What happens during independent segregation?
-The homologous pair contains one maternal chromosome and one paternal chromosome
-When the homologous pair is separated in meiosis 1, it is completely random which chromosome from each cell ends up in each daughter cell
-So the four daughter cells produced by meiosis have completely different combinations of those maternal and paternal chromosomes
When does independent segregation occur?
-Metaphase 1
What happens during anaphase 1?
-Spindle fibres connected to the chromosomes contract
-This causes homologous chromosomes to separate apart and are pulled to opposite poles
-Sister chromatids are kept together
What happens during telophase 1?
-spindle fibres break down
-Nuclear membrane forms
-around each set of chromosomes so there are 2 nuclei
-Cytokinesis occurs and 2 new cells are formed when the membrane pinches inward
What happens during prophase 2?
-The chromosomes condense and become fatter
-The chromosomes appear as a double structure consisting of 2 sister chromatids joined by a centromere
-Centrioles begin to migrate towards the poles
-The spindle fibres begin to form extending from the centrioles
-The nuclear membrane and nucleolus breaks down in late prophase/ early metaphase
-Chromatids become visible
What happens in metaphase 2?
-Centrioles reach the poles
-Microtubules extend between centrioles to form spindle fibres
-Bivalents line up on the equator and attach to the the centromere
-One homologous chromosome is attached by microtubules to one pole and the other is attached to the opposite pole
What happens in anaphase 2?
–Spindle fibres connected to the chromosomes contract
-This causes the centromeres to divide and sister chromatids separate creating two sister chromosomes
-Daughter chromosomes are pulled to opposite poles
What happens in telophase 2?
-spindle fibres break down
-Nuclear membrane forms
-around each set of chromosomes so there are 2 nuclei
-Cytokinesis occurs and 2 new cells are formed when the membrane pinches inward
-Chromosomes uncoil to form chromatin
What are centrioles?
-A pair of organelles that organises microtubules into spindle fibres on which chromosomes are separated when cells divide
What is the centromere?
-The region of a chromosome that holds sister chromatids together until they are separated at the beginning of anaphase
What is a chromatid?
-One of two identical threads of DNA that are joined at a centromere to form a complete chromosome after DNA replication
-After replication they become daughter chromosomes
What is chromatin?
-The substance that forms chromosomes consisting of DNA and associated proteins
What is cytokinesis?
-The division of a cytoplasm immediately following telophase
What is a diploid?
-A nucleus or cell that contains two complete sets of chomosomes one from each parent
What is a gamete?
-A haploid sex cell resulting from meiosis and is capable of using with another gamete to form a zygote
What is a haploid?
-A nucleus or cell with a single set of unpaid chromosomes
What are homologous chromosomes?
-A pair of similar chromosomes (one maternal, one paternal) which pair up in meiosis 1
What is independent assortment
-The chromosomes can align on the metaphase plate in different combinations
What is meiosis?
-The process of cell division underlying reproduction in which one diploid cell divides to form 4 haploid daughter cells
What are microtubules?
-Fibrous, hollow rods that function to help support and shape the cell
-They also function as routes along which organelles and cytoplasm can move using olecular motors
What are spindle fibres?
-Microtubules that form between a cells poles, coordinating the movement of chromosomes during cell divison
What is a zygote?
-A diploid cell formed from two haploid gametes during fertilisation
