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Meiosis And Sexual Reproduction Flashcards

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1
Q

What is sexual reproduction?

A

The production of a new individual resulting from the joining of two specialised cells known as gametes

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2
Q

What are the advantages and disadvantages of sexual reproduction?

A

• advantages:
- it increases genetic variation as a result of the fusing of gametes from two different individuals. In a changing environment this gives a greater chance that one of the offspring will have the combination of genes that will improve their chance of surviving
• disadvantages:
- it is not always easy to find a mate
- it is more expensive in terms of bodily resources as it usually involves special sex organs

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3
Q

What is a cell containing two full sets of chromosomes called?

A

Diploid (2n)

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4
Q

How is offspring prevented from having four sets of chromosomes in sexual reproduction?

A

Haploid (n) nuclei are formes with one set of chromosomes, usually within the specialised cells called gametes

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5
Q

What is fertilisation?

A

When two haploid nuclei fuse to form a diploid cell called a zygote

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6
Q

What are the sex organs often called in complex animals?

A

Gonads

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7
Q

What are the male and female sex organs in plants?

A

Male: anthers
Female: ovaries

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8
Q

Where are the male and female gametes formed in plants?

A
  • male gametes are formed in the pollen, produced in the anthers
  • femalw gametes are formed in the ovules contained in the ovaries
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9
Q

What are the male gonads in animals?

A

Testes

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10
Q

What are the female gonads in animals?

A

Ovaries

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11
Q

What are the male gametes in animals?

A

Spermatozoa produced in the testes

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12
Q

What are the female gametes in animals?

A

The ova produced in the ovaries

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13
Q

How can the male and female animal gametes be summarised?

A
  • male: many, mini, motile

* female: few, fat fixed

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14
Q

What is meiosis?

A

A reduction division that only occurs in the sex organs

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15
Q

How do the products of meiosis differ in animals and plants?

A
  • in animals the gametes are formed directly from meiosis
  • in plants meiosis forms special male cells called microspores and female cells called meagaspores which then produce the gametes
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16
Q

Desribe the stages of meiosis

A
  • prophase 1: each chromosome appear in the condensed form with two chromatids. Homogolous pairs of chromosomes associate with each other. Crossing over occurs
  • Metaphase 1: the spindle forms and the pairs of chromosomes line up on the metaphase plate
  • Anaphase 1: the centromeres do not divide. One chromosome (pair of chromatids) from each homogolous pair moves to each end of the cell. As a result the chromosome number in each cell is half that of the original
  • Telophase 1: the nuclear membrane reforms and the cells begin to divide. In some cells this continues to full cytokinesis and there may be a period of interphase. During this interphase there is no further replication of the DNA
  • prophase 2: new spindles are formed
  • metaphase 2: the chromosomes still made up pairs of chromatids line up on the metaphase plate
  • anaphase 2: the centromeres now divide and the chromatids move to the opposite ends of the cell
  • telophase 2: nuclear envelopes re-form, the chromosomes return to their interphase stare and cytokinesis occurs, giving four daughter cells each with half the number of chromosomes of the original diploid cell
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17
Q

What are the differences between mitosis and meiosis?

A
  • in prophase in mitosis the chromosomes replicate to form chromatids joined by a centromere. In meisosis the two chromosomes of each pair (known as homogolous pairs) stay close together and at this stage crossing over/ recombination takes place introducing genetic variation
  • the centromeres do not split in the first division of meoisis so pairs of chromatids move to the opposite ends of the cell and then immediately go into a second division like mitosis
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18
Q

Why is meiosis important?

A
  • it reduces the chromosome number in gametes from diploid to haploid so that sexual reproduction is possible without each generation carrying an increasing burden of genetic material
  • it is the main way in which genetic variation is introduced to a species
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19
Q

What are the two main ways genetic variation is introduced in meiosis?

A

• independent assortment (random assortment): the chromosomes that came from the individual’s two parents are distributed into the gametes and so into their offspring completely at random. There are eight million potential genetic combinations within the sperm or egg
• crossing over (recombination) this process takes place when large multu enzyme complexes cut and join bits of the maternal and paternal chromatids together. The points where the chromatids break are called chiasmata. These are important in two ways:
- the exchange of genetic material leads to added genetic variation
- errors in the process lead to mutation and this is a further way of introducing new combinations into the genetic makeup of a species

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20
Q

One of the most common forms of a chromosome mutation is translocation. What is this?

A

When a piece from one pair of homogolous chromosome breaks off and reattaches to one of a completely different pair of chromosomes

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21
Q

What are balanced and unbalanced translocations?

A
  • Balanced: a piece is effectively swapped between two different chromosomes. People who have balanced translocations are often healthy
  • unbalanced: one chromosome loses a piece and another chromosome gains it. These mutations can cause big changes in the phenotype of the individual
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22
Q

What is the non-disjuction of chromosomes?

A

During the reduction division of meiosis the members of one of the homogolous pairs of chromosomes fail to seperate during anaphase 2. As a result one of the gametes has two copies of that chromosome and the other has no copies

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23
Q

What is aneuploidy and what are the different types of aneuploidy?

A
  • when a cell either lacks a whole chromosome or has more than two of a chromosome
  • monosomy is when a gamete with no copies of a chromosome joins with a normal gamete and is fertilised to produce a cell with only one of the homogolous pair present
  • polysomy is when a gamete with two copies of the chromosme joins with a normal gamete and is fertilised to produce a cell with three or more rather than two chromosomes of a particular type
24
Q

What is Down’s syndrome?

A
  • there is non-disjuction of chromosome 21
  • after fertilisation with a normal gamete the resulting zygote will have polysomy, with three copies of chromosome 21
  • babies born with trisomy of chromosome 21 have down’s syndrome
  • the extra chromosome affects both mental and physical development
  • children born with down’s syndrome will often have problem’s including heart abnormalities, learning difficulties, lack of muscle tone and visual problems
25
Q

Which chromosome is less unusual and life threatning if there is aneuploidy of it?

A

A sex chromosome

26
Q

How does the presence or absence of a y chromosome determine the route for sexual development in human embryos?

A

Any embryo with at least one Y chromosome ajll develop male characteristics while any embryo lacking a Y chromosome will develop female characteristics

27
Q

What is Turner’s syndrome?

A
  • when there is non-disjuction of the male sex chromosomes an egg may be fertilised by a sperm that has no sex chromosomes. The resulting embryo will have monosomy and have the genotype XO
  • the affected person is female but she is infertile and will not undergo puberty without being given extra sex hormones
28
Q

What is Kleinfelter’s syndrome?

A
  • non-disjunction of the male chromosomes that gives a sperm carrying both an X and Y chromosome. If one of the sperm fertilises a normal egg the resulting embryo will be XXY
  • affected individuals have small testes and produce little testosterone. They have little facial and body hair, may develop breast tissue, have less muscle development than usual and may be infertile
29
Q

What is the process of forming gametes called?

A

Gametogenesis

30
Q

How do both mitosis and meiosis play a role in gametogenesis in mammals?

A

Mitosis produces the precursor cells, meiosis brings about the reduction divisions that result in gametes

31
Q

When does the process of gameteogenesis take place in human males and females?

A
  • in males the process continues constantly from puberty.
  • in females the mitotic divisions take place before birth. the meiotic divisions take place in a few oocytes in each monthly cycle from puberty to menopause and are only completed if the oocyte is fertilised
32
Q

What is spermatogenesis?

A

The formation of spermatozoa. Each primordial germ cell in the testes results in large numbers of spermatozoa. There are enormous numbers of primordial germ cells in the testes producing millions of spermatozoa on a regular basis

33
Q

What happens in the process of spermatogenesis?

A
  • the diploid primordial germ cell divides several times by mitosis to form diploid spermatogonia
  • the spermatogonia then grow without further division until they are big enough to be called primary spermatocytes
  • the spermatocytes undergo meiodis. The first meiotic division results in two haploid cells called secondary spermatocytes
  • the second meiotic division results in four haploid cells called spermatids
  • the spermatids then differentiate in the tubules of the testes to form spermatozoa, the active gametes capable of fertilising an ovum
34
Q

What is oogenesis?

A

• the formation of ova. Each primordial germ cell in the ovary results in only one ovum. As a result the number of female ova is always substantially smaller than the number of spermatozoa. Ova contain a much higher proportion of resources than sperm so there is greater investement of resources in each one. It would not make bioloigical sense to waste resources by producing too many of them

35
Q

How does the process of oogenesis take place?

A
  • the diploid primordial germ cell divides several times by mitosis to form diploid oogonia. Most of the oogonia do not develop further. They simply degenerate. Only one continues to grow and substantial amounts of storage material go into the cell making it very large compared with spermatocytes. At this stage the large cell is called the primary oocyte
  • the oocyte undergoes meiosis. The first meiotic division results in two cells of very unequal size. The larger cell is the secondary oocyte. The other, much smaller cell sticks to the oocyte and is called the first polar body. At this stage the oocytes do not divide further until after ovulation.
  • the second meiotic division is only completed after fertilization occurs. The secondary oocye divides to form the haploid ovum and another polar body, whilst the first polar body divides to form two more polar bodies. The polar bodies do not have any function except to recieve the chromosomes in meiotic divisions. They degenerate and die as the ovum develops
36
Q

What are the characteristics of spermatozoa?

A
  • they must remain in suspension in the semen so they can be transported through the female reproductive tract.
  • they must be able to penetrate the protective barrier around the ovum and deliver the male haploid genome safely inside
37
Q

What is the structure of a spermatozoa?

A

They have:
• acrosome - membrane bound storage site for enzymes that digests the layers surrounding the ovum and allow’s the sperms head to penetrate
• nucleus - contains the highly condensed chromosomes. The condensed state of the genetic material reduces the amount of energy needed to transport it
• mitochondria- tightly packed into the middle section of the sperm provide the ATP for the lashing of the tail
• microtubules - produce the whip-like movements of the tail that keep the mature sperm in suspension and help it ‘swim’ towards the ovum
• tail - a flagellum that propels the sperm by its movement in a liquid environment

38
Q

What are the characteristics of ova?

A
  • they can greatly vary in size depending on the animal
  • they usually contain food for the egg
  • in birds and reptiles a lot of development takes place before the animal hatches, so the egg contains a large food store
  • in mammals once the developing fetus has implanted in the uterus it is supplied with nutrients from the blood supply of the mother and so large food stores in the egg are unnecessary
39
Q

What is the structure of an ova?

A
  • they have a cytoplasm with many food reserves
  • they have a cell surface membrane
  • they are surrounded by a zona pellucida - a clear ‘jelly-like’ layer
40
Q

Why is the formation of gametes in flowering plants more complex?

A

Because plants have two pahses to their life cycles:
• the sporophyte generation is diploid and produces spores by meiosis
• the gametophyte generation that results is haploid and give rise to the gametes by mitosis
• the main body of the plant we see is the diploid sphorophyte
• the haploid gametophytes are reduced to parts of the contents of the anther and the ovary. They are produced by meiosis from spore mother cells

41
Q

How is pollen formed?

A
  • meiosis occurs in the anthers resulting in vast numbers of pollen grains that carry the male gametes
  • each anther contains four pollen sacs where the pollen grains develop.
  • in each pollen sac there are large numbers of microspore mother cells. These are diploid. They divide by meiosis to form haploid microspores which are the gametophyte generation
  • the gametes are then formed from the microspores by mitosis. They contain two haploid nuclei: the tube nucelus and generative nucleus
  • the tube nucleus produces the pollen tube that penetrates through stigma, style amd ovary and into the ovule.
  • the generative nucleus then fuses with the nucleus of the ovule to form a new individual
42
Q

How is the egg cell formed in flowering plants?

A
  • the diploid megaspore mother cell is produced by mitosis
  • diploid megaspore mother cells divide by meiosis to give rise to four haploid megaspores, three of which disintegrate leaving one to develop
  • the megaspore undergoes three mitotic divisions that result in an embryo sac containing an egg cell, two polar nuclei and various other small cells
43
Q

What is the embryo sac in a plant covered by?

A

The ovule which then has a layer of tissue called the nucellus around it

44
Q

What is the ovule attached to the wall of the ovary by?

A

A pad of special tissue called the placenta

45
Q

How is pollen transferred from one plant to another for pollination to take place?

A
  • some flowers attract other organisms such as insects, birds or mammals to transfer the pollen from one plant to another
  • others rely on the wind to carry their pollen from plant to plant
46
Q

How do animals make sure their gametes meet?

A
  • External fertilisation: occurs outside the body, with the female and male gametes shed directly into the environment where they meet and fuse. This is common onyl in aquatic species as sperm and ova are very vulnerable to drying. It is largely a matter of chance whether fertilisation takes place. More complex animals have evolved rituals that increase the likelihood of fertilisation by ensuring that the ova and slerm are released at the same time close to eachother. However often gametes do not meet so external fertilization is very wasteful
  • internal fertilisation: involves the transfer of the male gametes directly to the female. This makes fertilisation more likely. In mant species the male produces packages of sperm for the female to pick up and transfer to her body. More complex animals have evolved a system where male gametes are released directly into the body of the female during mating. This makes sure that the ova and sperm are kept in a moist environment and are placed as close together as possible which maximises the chances of successful fertilisation
47
Q

What enables the sperm to be able to penetrate the ovum as it moves through the reproductive tract?

A

The acrosome region matures as sperm move through the female reproductive tract

48
Q

What happens in fertilisation in humans?

A
  • the ovum released at ovulation is really a secondary oocyte with one polar body. It is surrounded by the zona pellucida and some of the follicle cells
  • many sperm cluster round the ovum and as soon as the heads of the sperm touch the surface of the ovum the acrosome reaction is triggered. Enzymes are released from the acrosome which digest the follicle cells and zona pellucida
  • one sperm alone does not have enough resources to penetrate the protective layers so this is one reason for the large amounts of sperm produced in ejaculation
  • eventually one sperm will get through the weakened protective barriers and touch the surface membrane of the oocyte
  • this has several instantaneous effects:
  • the oocyte completes its second meiotic division producing a haploid egg nucleus to fuse with the haploid male nucleus
  • ion channels in the cell membrane open and close so that the inside of the cell instead of being electrically negative with respect to the outside becomes positive. This alteration blocks the entry of any further sperm so that no more can fertilise the egg which would result in polyspermy
  • it is a temporary measure until a tough fertilisation membrane forms around the ovum which will then repel other sperm
  • the head of the sperm enters the oocyte but the tail region is left outside. Once the head is inside it absorbs water and swells, releasing it’s chromosomes to fuse with those of the ovum forming a diploid zygote. At this point fertilisation has occured
49
Q

How does fertilisation take place in flowering plants?

A

• the male gamete is contained within the pollen grain and the female gamete is embedded deep within the tissue of the ovary
• the pollen grain lands on the surface of the stigma of the flower during pollination
• the molecules on the surface of the pollen grain and stigma interact. If they recognise eachother as being from the same species the pollen grain begins to grow or germinate
• a pollen tube begins to grow out from the tube cell of the pollen grain through the stigma into the style
• the tip of the pollen tube produces hydrolytic enzymes to digest the tissue of the style, so the pollen tube can make it’s way down between the cells.
• the digested tissue acts as a nutrient source for the pollen tube as it grows
• as the pollen grows down towards the ovary the generative cell containing the generative nuclei travels down it. The nucleus of this cell divided by mitosis as it moves down the tube to form two male nuclei.
• the pollen tube grows through the ovary to reach an ovule and eventually the tip of the pollen tube passes through the micropyle to the ovule. The growth of the pollen tube is very fast due to the rapid elongation of the cell
• once the tube has entered the micropyle the two male nuclei are passed into the ovule so that fertilisation can occur
• flowering plants undergo double fertilisation:
- one male nucleus fuses with the two polar nuclei to form the triploid endosperm nucleus which is involved in supplying the embryo plant with food when it begins to germinate
- the other male nucleus fuses with the egg cell to form the diploid zygote. At this point fertilisation is complete

50
Q

In humans the zygote is said to be totipotent. What does this mean?

A

It has the potential to form all of the 216 different types needed for an entire new person

51
Q

What is cleavage?

A
  • the first stage of the embryo development
  • cleavage involves a special type of mitosis in which cells divide repeatedly without the normal interphase for growth between the divisions.
  • this happens as the embryo travels down the oviduct.
  • the result of cleavage is a mass of small, identical and undifferentiated cells forming a hollie sphere known as a blastocyst
  • in humans this process takes about 5-6 days
52
Q

How long from ovulation does it take for an ovum to reach the uterus (via the oviduct)

A

About 80 hours

53
Q

What are the tiny cells of the early human embryo known as?

A

The embryonic stem cells

54
Q

What are stem cells?

A

Undifferentiated cells with the potential to form many different cells. The earliest cells in an embryo are totipotent like the zygote

55
Q

What do the cells do in the blastocyst?

A
  • the outer layer of cells goes on to form the placenta
  • the inner layer of cells have already lost some of their ability to differentiate. They can form almost all types of cell needed but not tissue such as the placenta. These cells are known as pluripotent embryonic stem cells
56
Q

What is cell determination?

A

The predestination of cells to become particular types of tissue from early in development of the embryo

57
Q

What is cell determination closely linked to?

A

The position of the cells in the embryo

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