Flashcards in Meiosis Deck (36):
What happens in meiosis 1 prophase?
1. Early prophase 1 is as mitosis
2. Middle prophase 1, homologous chromosomes pair up. This is called synapsis and each pair is called a bivalent
3. Centrosomes moving to opposite ends of nucleus as in mitosis
4. Late prophase 1
-Nuclear envelope breaks up as in mitosis
-Crossing over of chromatids may occur
-Nucleolus 'disappears' as in mitosis
5. At the end of prophase 1 a spindle is formed
What is a bivalent pair?
1. Chromatids may break and may reconnect to another chromatid
2. One or more chiasmata may form, anywhere along length
What is a chiasma?
Point where crossing over occurs (plural: chiasmata)
What happens in metaphase 1?
1. Bivalents line up across equator, attached by centromeres
2. Spindle is formed as in mitosis
What happens in anaphase 1?
1. Centromeres do not divide, unlike in mitosis
2. Whole chromosomes move towards opposite ends of single, centromeres first, pulled by microtubules
What happens in telophase 1?
1. Nucelar envelope reformed, nucleus re-froming, cytokinesis, remains of spindle, chromosomes have reached poles of spindle (as in mitosis)
2. Animal cells usually divide before entering meiosis 2. Many plant cells usually go straight into meiosis 2 with no reformation of nuclear envelopes or nucleoli. During meiosis 2, chromatids separate as in mitosis
What happens in prophase 2?
1. Nuclear envelope and nucleolus disperse
2. Centrosomes and centrioles replicate and move to opposite poles of the cell
What happens in metaphase 2?
1. Chromosomes line up separately across the equator of spindle
What happens in anaphase 2?
1. Centromeres divide and spindle microtubules pull the chromatids to opposite poles
What happens in telophase 2?
1. This is like telophase of mitosis, but in meiosis telophase 2 four haploid daughter cells are formed
What is a diploid cell?
A diploid cell is one that poses two complete sets of chromosomes; the abbreviation for diploid is 2n
What is a haploid cell?
A haploid cell is one that possesses one complete set of chromosomes; the abbreviation for haploid is n
What is meiosis?
1. An animal cell is shown where 2n=4, and different colour represent maternal and paternal chromosomes
2. The associated behaviour of the nuclear envelope, cell surface membrane and centrosomes is also shown and remember that each centrosome contains a pair of centrioles
How is mitosis and meiosis different?
1. Meiosis involves two divisions, called meiosis I and meiosis II
What is meiosis I and II?
1. Meiosis I is a reduction division, resulting in two daughter nuclei with HALF the number of chromosomes of the parent nucleus
2. In Meiosis II, the chromosomes behave as in mitosis so that each of the two haploid daughter nuclei divides again
-Meiosis therefore results in a total of four haploid nuclei
-Note that it is the behaviour of the chromosomes in meiosis I that is particularly important and contrasts with mitosis
How does meiosis produce genetic variation?
-Two of the events that take place in meiosis help to produce genetic variation between the daughter cells that are produced, these are:
1. Independent assortment of the homologous chromosomes
2. Crossing over, which happens between the chromatids of homologous chromosomes
What is genetics and meiosis?
1. When these genetically different gametes fuse, randomly at fertilisation, yet more variation is produced amongst the offspring
2. In order to understand how these event produce variation, we first need to consider the genes hat are careered on the chromosomes, and the way in which these are passed on from parents to offspring, and this branch of biology is known as genetics
How many chromosomes are in the daughter cells in mitosis and meiosis? What is the genetic definition?
1. Mitosis: 46, diploid
2. Meiosis: 23, haploid
What are the types of cell produced in mitosis and meiosis?
1. Mitosis: cells required for growth, maintenance and repair
2. Meiosis: sex cells (sperm or egg cells)
How many times does cell division occur in mitosis and meiosis?
1. Mitosis: once
2. Meiosis: twice
How many daughter cells are produced in mitosis and meiosis?
1. Mitosis: two
2. Meiosis: four
Is there a formation of chiasma is mitosis and meiosis?
1. Mitosis: no
2. Meiosis: yes
Why re there two different ways a cell can divide?
1. When a cell is preparing to divide, it first copies everything inside it, including your DNA
2. Your DNA is divided into 46 chromosomes and is a mixture of information from your parents ands you have 23 chromosomes from your father and 23 from your mother
3. Every cell in your body has 46 chromosomes apart from your red bloods cells (that don't contain DNA) and your gametes
4. These cells (sperm in males or eggs in females) only have 23 chromosomes
5. This means that during fertilisation, the a sperm cell fuses with an egg cell, the fertilised egg has 46 chromosomes in total
6. Every other cell type that forms form the fertilised egg as it develops Ito an embryo therefore has 46 chromosomes
7. If sperm and egg cells has 46 chromosomes each, then the cell formed when they fused would have 92 chromosomes in total, and would die
-So you have two groups of cells in your body: those with 23 chromosomes (sperm or egg cells) or those with 46 chromosomes (most other cell types)
-Cells with 23 chromosomes are referred to as haploid, whereas cells with 46 chromosomes are referred to as diploid
-Because these cells have different numbers of chromosomes, the way in which they divide is also different ad this is why there are two different ways a cells can divide: mitosis maintains the number of chromosomes in a cell, meiosis halves the number
What are homologous chromosomes?
Homologous chromosomes are a pair of chromosomes in a diploid cell that have the same structure as each other, with the same genes (by not necessarily the same alleles of those genes) at the same loci, and that pair together to form a bivalent during the first division of meiosis
What is a gene?
A gene is a length of DNA that codes for a particular protein or polypeptide
What is an allele?
An allele is a particular variety of a gene
What is a locus?
A locus is the position at which a particular gene is found on a particular chromosomes; the same gene is always found at the same locus
What is gametogensis?
1. In animals such as humans, meiosis occurs as gametes are formed inside the rested and ovaries
2. The formation of male gametes is know as spermatogensis
3. The formation of female gametes is known as oogenisis
Where does sperm production occur?
1. Sperm production takes places inside tubules in the testes
2. Here, diploid cells divide by mitosis to produce numerous diploid spermatogonia, which grow to form diploid primary spermatocytes
3. The first division of meiosis then takes place, forming two haploid secondary spermatocytes
4. The second division of meiosis then produces haploid spermatids, which mature into spermatozoa
What happens in oogenesis?
1. Oogenesis follows a similar pattern, but many fewest gametes are made than during spermatogenesis, and the process takes much longer with ‘waiting Stages’
2. It takes place inside the ovaries, where diploid cells divide by mitosis to produce many oogonia
3. This begin to divide by meiosis, but stop when they reach prophase 1
4. At this stage they are called primary oocytes, and they are of course still diploid
-All of this happens before a baby girl is born, and at birth she has around 400,000 primary oocytes in her ovaries
What happens when girls reach puberty with their oocytes?
1. When she reaches puberty, some of her primary oocytes get a little further with their division by meiosis
2. They proceed from prophase 1 to the end of the first meiotic division, forming two haploid cells
3. However, the division is uneven: one cell gets most of the cytoplasm, and becomes a secondary occyte, while the other is little more than a nucleus, and this is called a polar body
4. The polar body can be thought of as simply a way of getting rid of half of the chromosomes, and has no further role to
Play in reproduction
What happens each month with her ovaries?
1. Each month, one secondary oocyte is released into the oviduct from one of the ovaries
2. If it is fertilised, it continues it’s division by meiosis, and can now be called an ovum
3. The chromosomes of the spermatozoan and the ovum join together to form a single diploid nucleus, and the cell that is made by this process is called a zygote
4. The zygote can now divide repeatedly by mitosis to form first an embryo, then a fetus
Describe gametogenesis in flowering plants?
1. Male gametes are pricked in the anthers
2. Female gametes are produced in the ovules
What happens inside the anthers of plants?
1. Inside the anthers, diploid pollen mother cells divide by meiosis to from four haploid cells
2. The nuclei of each of these haploid cells then divide by mitosis, but the cell itself does not divide (cytokinesis does not take place), resulting in cells that each contain two haploid nuclei
3. These cells mature into pollen grains, each surrounded by a protective wall made up of a tough exine and thinner intine
4. One of the haploid nuclei is called the tube nucleus, and the other is generative nucleus
5. These are the male gametes
What happens in the ovules of plants?
1. Inside each ovule, a large diploid, spore mother cell develops
2. This cell divides by meiosis to produce four haploid cells
3. All but one of these degenerates, and the one surviving haploid cell develops into a embryo sac
4. The embryo sac grows larger, and its haploid nucleus divides by mitosis three times, forming EIGHT haploid nuclei
5. One of these becomes the female gamete