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Biology > 3.3 > Flashcards

3.3 Flashcards

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1
Q

Name the sex organs where meiosis occurs and the gametes produced in animals and plants

A

animals
male: testes—> sperm
female: ovaries—>eggs/ova

plants
male: anther—> pollen
female: ovary—> ovules

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2
Q

What happens in meiosis?

A

one diploid nuclei divides to produce four haploid nuclei

often called a reduction division because meiosis produces cells w/ half the number of chromosomes as the parent cell

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3
Q

Sexual life cycles

A

the halving of chromosome number allows a sexual life cycle with fusion of gametes

in order to reproduce diploid organisms need to make haploid gametes, so that fertilisation results in the formation of a diploid zygote

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4
Q

What happens before meiosis?

A

interphase
- DNA is replicated during S-phase
- Chromosomes aren’t supercoiled so are not visible as individual structures
-Nucleolus is present

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5
Q

Explain what happens during meiosis 1 prophase 1 and draw a diagram

A

Early prophase 1:
- DNA supercoils and chromosomes become visible as 2 sister-chromatids
- Nucleolus disappears
- centrioles begin to move to opposite poles
- nuclear membrane still present

mid-prophase 1:
- homologous chromosomes pair up in a process called synapses, forming a bivalent

late prophase 1:
- crossing-over occurs between non-sister chromatids in a bivalent
- non-sister chromatids break and rejoin at chiasmata
- this generates new combinations of alleles
- nuclear membrane disintegrates

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6
Q

What happens during meiosis 1 metaphase 1? draw a diagram

A

centrioles are at opposite poles and spindle fibres have formed between them

bivalent are lined up on the equator of the cell and spindle fibres are attached to each chromosome of the pair at their centromeres

the orientation of the bivalent son the metaphase plate is random

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7
Q

What happens during meiosis 1 anaphase 1? draw a diagram

A
  • spindle fibres shorten and pull homologous chromosomes towards opposite poles of the cell
  • the bivalent a are separated, however each chromosome still consists of 2 sister chromatids joined at their centromeres
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8
Q

What happens during meiosis 1 telophase 1? draw a diagram

A
  • spindle fibres disintegrate
  • new nuclear membranes form around each set of chromosomes at opposite poles
  • each nucleus now contains only one chromosome from each homologous pair, so haploid cells have been produced
  • DNA decondenses
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9
Q

What happens during meiosis 2 prophase 2? draw a diagram

A
  • DNA supercoils
  • Individual chromosomes become visible as 2 sister chromatids joined together at the centromere
  • centrioles move to opposite poles
  • nuclear membrane disintegrates
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10
Q

What happens during meiosis 2 metaphase 2? draw a diagram

A
  • spindle fibres form between the centrioles on opposite poles of the cell
  • chromosomes lines up along the equator of the cell attached to spindle fibres by their centromere
  • random orientation
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11
Q

What happens during meiosis 2 anaphase 2? draw a diagram

A
  • centromeres divide
  • spindle fibres shorten and pull the chromatids (now called chromosomes) to opposite poles of the cell
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12
Q

What happens during meiosis 2 telophase 2? draw a diagram

A
  • spindle fibres disintegrate
  • chromosomes decondense
  • new nuclear membranes are formed around each set of chromosomes
  • 4 GENETICALLY DIFFERENT HAPLOID CELLS HAVE BEEN PRODUCED
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13
Q

Describe how crossing over can give rise to genetic variation

A
  • in prophase 1 of meiosis 1 homologous chromosomes pair up to form bivalents
  • non-sister chromatids break and rejoin at precisely the same point (chiasmata)
  • this leads to exchange of alleles between them
    1 can occur at random positions anywhere along the chromosome and the longer the chromosome the more chiasmata might form
    —> GENERATES NEW COMBINATION OF ALLELES
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14
Q

Describe how random orientation of bivalents can give rise to genetic variation

A
  • in metaphase 1 of meiosis 1, bivalents attach to spindle fibres by their centromeres and move to the cells equator
  • the orientation of the homologous chromosomes in a bivalent is random and independent of each other
  • so each homologous chromosome has an equal chance of being pulled to either pole during anaphase 1
    —> LEADS TO INDEPENDENT ASSORTMENT OF HOMOLOGOUS CHROMOSOMES
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15
Q

Describe how fertilisation can give rise to genetic variation

A
  • fusion of gametes from different parents generates genetic variation
  • 2 haploid cells fuse to form 1 diploid zygote
  • this allows alleles from 2 different individuals to be combined into an individual
  • fusion of gametes is random
  • there is an almost infinite number of possibilities for genetic variation
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16
Q

What happens in non-disjunction?

A
  • in anaphase 1 of meiosis 1, homologous chromosomes of a bivalent may fail to separate so both are pulled to the same pole
    (can also happen during anaphase 2 of meiosis 2 if sister chromatids fail to separate)
17
Q

What is the result of non-disjunction?

A
  • gametes will be produced that either have an extra chromosome (24), or that lack a chromosome (21)
  • in humans this means that if a gamete produced by non-disjunction is fertilised the zygote will have 47 or 45 chromosomes
18
Q

How does non-disjunction cause down syndrome and other chromosomal abnormalities?

A
  • down syndrome is due to non-disjunction of chromosome 21
  • the affected individual has one extra copy of chromosome 21 (3 instead of 2) which is called trisomy 21
  • symptoms include: hearing loss, heart and vision disorders, some mental and growth problems
19
Q

How can age of parents influence chances of non-dishunction?

A
  • there is a strong correlation between increasing maternal age and the incidences of non-disjunction events (chromosomal abnormalities)
  • there is ~1% probability of a mother aged 40 giving birth to a child with down syndrome
20
Q

Describe the method of chorionic villus sampling and the associated risks

A
  • it involved taking a sample from chorionic villi (placenta tissue) via a catheter inserted through the cervix, and guided by an ultrasound
  • can be done at ~11 weeks of pregnancy
  • has a slight risk of miscarriage (~1%)
21
Q

Describe the method of amniocentesis to obtain cells for karyotyping, and the associated risks

A
  • it involves taking a sample of amniotic fluid (contains fetal cells for karyotyping) using a syringe (guided by ultrasound)
  • can be done at ~16 weeks of pregnancy
  • has a lower risk of miscarriage (~0.5%)

Biology (38 decks)

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