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Biology HL > Genes And Cell Division > Flashcards

Genes And Cell Division Flashcards

1
Q

What is cell division?

A

new cells are produced by the division of pre-existing cells. The cell that divides is the mother cell and the two cells produced after the division are the daughter cells

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2
Q

What is Cytokinesis

A

cytokinesis is the division of a cell’s cytoplasm to form two cells. It occurs after mitosis and happens differently in plant and animal cells

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3
Q

What happens to plant cells in cytokinesis?

A

plants make a new membrane (the cell plate) across the cell’s equator with plasma membrane on both sides, this divides the cell in two. Vesicles containing pectin are linked up across the equator to form a middle lamella. Cellulose is then added on either side to form two walls, one for each daughter cell

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4
Q

What happens to animal cells in cytokinesis?

A

Animals divide the cytoplasm of cells by moving the plasma membrane. Movement is due to actin and myosin proteins adjacent to the membrane. Before cytokinesis, they are randomly arranged, but some are reorientated so they run in parallel in a ring around the equator of the cell, where they exert tension to form a cleavage furrow, with the membrane pulled inwards so it eventually splits the cell.

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5
Q

Unequal and equal cytokinesis

A

In many cases, cytokinesis divides the cytoplasm of the mother cell into equal halves. This happens for example when a human zygote divides to form a two-cell embryo. Cytoplasm is sometimes divided unequally. Small cells produced by unequal division can survive and grow if they receive a nucleus plus at least one mitochondrion and other organelle that cannot be assembled from components in the cell.

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6
Q

What is mitosis used for?

A

Mitosis- for continuity

With mitosis, daughter cells receive all the chromosomes and genes of the mother cell. The chromosome number is unchanged

Mitosis is used in asexual reproduction to produce genetically identical offspring. It is also used in multicellular organisms to produce genetically identical body cells

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7
Q

What is meiosis used for?

A

Meiosis- for change

In meiosis a diploid nucleus divides into haploid nuclei, halving the chromosome number. This allows haploid gametes to be produced in sexual life cycles

Meiosis generates genetic diversity because every haploid cell produced from a diploid mother cell has a different combination of alleles

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8
Q

Why is DNA replicated before Mitosis and Meiosis I

A
  • Cells replicate all their DNA before the start of both mitosis and meiosis. DNA replication ensures that two daughter cells produced by mitosis will receive the entire genome
  • DNA replication provides enough DNA for a mother cell to divide twice in meiosis, producing four haploid cells. It also provides enough DNA for recombination by the process of crossing over
  • The DNA is in an elongated state when it is replicated and is then packed up tightly (condensed) during the early phases of mitosis or meiosis. Condensation makes the two DNA molecules visible as separate structures. They are called sister chromatids. Throughout the early phases of mitosis and meiosis, each chromosome consists of two sister chromatids. The chromatids only separate in the penultimate phase
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9
Q

What happens during interphase of mitosis?

A
  • longest and most variable of the phases
  • G1 phase- cell grows, DNA is transcribed + protein synthesis
  • S phase- DNA replication
  • G2 phase- cell grows, organelles increase in number, DNA condenses from chromatin → chromosomes + microtubules begin to form
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10
Q

What happens during prophase of mitosis?

A
  1. Chromatin fibres supercoil and condense to form chromosomes
  2. Nuclear envelope breaks down
  3. Spindle microtubules form
  4. Centromere of each chromosome has a region called the kinetochore that attaches the spindle
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11
Q

What happens during metaphase of mitosis?

A
  1. Chromosomes move towards the equator of the cell (metaphase plate)
  2. Centromeres lie on the plate
  3. The movement of chromosomes is due to spindle
  4. Centrosomes are now at the opposite poles
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12
Q

What happens during anaphase of mitosis?

A
  1. Sister chromatids of each chromosome are split
  2. The chromatids (now chromosomes), move to the opposite poles of the cell
  3. Chromatid movement is due to shortening of microtubules
  4. Each pole has a complete set of identical chromosomes

*n.b. anaphase is the shortest phase of mitosis

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13
Q

What happens during telophase of mitosis?

A
  1. The chromosomes are at each pole
  2. Nuclear envelope is reformed
  3. Chromosomes start to elongate to form chromatin
  4. Spindle apparatus disappears
  5. Cell elongates for cytokinesis
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14
Q

What happens during cytokinesis post mitosis?

A
  • in animals a cleavage furrow, a contractile ring of microfilaments is formed between the cells, the cells then split into two genetically identical daughter cells (diploid)
  • in plants a cell plate is formed by vesicles, this forms a new cell wall, the cells then split into two genetically identical daughter cells (diploid)
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15
Q

Outline the process of mitosis (7)

A
  1. chromosomes supercoil and condense
  2. the nuclear membrane breaks down
  3. growth of spindle fibres/ microtubules
  4. the spindle fibres attach themselves to chromosomes
  5. chromosomes line up at the equator
  6. the centromeres divide
  7. separation of the sister chromatids, chromosomes move to opposite poles
  8. reformation of the nuclear membrane around chromosomes at each pole
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16
Q

What are cyclins?

A
  • cyclins are proteins involved in coordinating the phases of the cell cycle
  • cyclin bind to enzymes called cyclin-dependent kinases to activate them
  • they switch on other proteins by adding phosphate groups to these proteins
  • if the cyclin concentration does not reach the threshold, the next phase of the cell cycle does not start
  • this allows cell division to be controlled
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17
Q

NOS- who discovered cyclins and how did they discover them?

A
  • Using sea urchin eggs, Tim Hunt discovered a protein that increases in concentration after fertilisation and then decreases
  • It was noted through experiments that there were repeated increases and decreases that occurred at the same time as phases in the cell cycle
  • Hunt named the proteins cyclins
  • Serendipity- Hunt did not set out to discover how the cell cycle is controlled
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18
Q

what is the evidence that meiosis may have evolved from mitosis?

A

The fact that DNA replication does occur suggests that meiosis evolved from mitosis (where initial DNA replication is necessary)

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19
Q

What is the benefit of DNA replication before meiosis I?

A

One benefit of the duplication of chromatids is that it increases the potential for genetic recombination to occur (more variation)

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20
Q

What happens during S phase interphase of meiosis?

A
  • Interphase is an active period that occurs before meiosis and involves key events needed to prepare the cell for successful division
  • DNA is replicated during the S phase of interphase, resulting in chromosomes that contain two identical DNA strands
  • These genetically identical strands are called sister chromatids and are held together by a region called the centromere
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21
Q

What happens in the first meiotic division?

A

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated

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22
Q

Describe the process of prophase I (meiosis)

A
  • chromosomes condense, the nuclear membrane dissolves, homologous chromosomes form bivalents, and crossing over occurs.
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23
Q

Describe the process of metaphase I (meiosis)

A

Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
the orientation of pairs of homolgous chromosomes is random

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24
Q

Describe the process of anaphase I (meiosis)

A

Centromeres do not divide
spindle fibre pulls the whole centromere with two sister chromatids to opposite poles

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25
Q

Describe the process of Telophase I (meiosis)

A

Arrival of centromere with sister chromatids at opposite poles
Chromosomes de-condense, the nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

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26
Q

What happens in the second meiotic division?

A

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I)

27
Q

Describe the process of prophase II (meiosis)

A
  • Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
28
Q

Describe the process of metaphase II (meiosis)

A

Spindle fibres from opposing centrosomes attach to chromosomes (at the centromere) and align them along the cell equator

29
Q

Describe the process of anaphase II (meiosis)

A

Spindle fibres contract and separate the sister chromatids, and chromatids (now called chromosomes) move to opposite poles

30
Q

Describe the process of telophase II (meiosis)

A

Chromosomes de- condense, nuclear membrane reforms, cells divide to form four haploid daughter cells; these cells may all be genetically distinct if crossing over occurs in prophase I (causing recombination of sister chromatids)

31
Q

How does meiosis generate variation?

A
  1. Random orientation of bivalents (metaphase I)
  2. Independent assortment of genes (metaphase I)
  3. Crossing over (prophase I)
32
Q

How does random orientation of bivalents generate variation?

A

a bivalent is a pair of homologous chromosomes, one inherited from the male and one from the female parent. Orientation of each bivalent in metaphase I determines which pole each chromosome moves to. Orientation is random and does not influence other bivalents, so many different combinations can be produced when homologous chromosomes separate in anaphase I.

33
Q

How does independent assortment of genes generate variation?

A
  • Independent assortment describes how pairs of alleles separate independently from one another during gamete formation
  • According to independent assortment, the inheritance of one gene/trait is independent of another
  • independent assortment is due to the random orientation of pairs of homologous chromosomes in meiosis I
  • During metaphase I, homologous chromosomes line up at the equator as bivalents in one of two arrangements
    - Maternal copy left/ paternal copy right OR paternal copy left/ maternal copy right
  • The orientation of each homologous pair is random and is not affected by the orientation of any other homologous pair
  • This means an allele in one chromosome has an equal chance of being paired with, or separated from, any allele on another chromosome (their inheritance is independent of one another).

Independent assortment will not occur two genes are located on the same chromosome (linked genes)

34
Q

How does crossing over in prophase I generate variation

A
  • Homologous chromosomes pair up in the very early stages of meiosis and non-sister chromatids exchange lengths of DNA by crossing over. This produces chromatids with new combinations of alleles. It is a significant source of genetic variation because where along the length of the chromosomes the exchange occurs is random.
  • when chiasmata form between bivalents in prophase I, DNA can be exchanged between non-sister homologous chromatids
  • the exchange of genetic material is called crossing over and produces new allele combinations on the chromosomes
  • these chromosomes that consist of genetic material from both homologues are called recombinant chromosomes
  • crossing over results in new combinations of alleles in haploid cells and thus increases the genetic diversity of potential offspring
35
Q

What are the differences between meiosis and mitosis?

A

Mitosis
occurs in/ produces somatic cells
one cell division
produces two daughter cells
daughter cells identical to parent cell/ does not produce genetic variation
produces cells for growth/repair
chromosome number stays the same/2n/diploid
pairing of chromosomes does not occur
no exchange of material between chromosomes/ no crossing over

Meiosis
occurs in/ produces sex cells
two cell divisions
produces 4 daughter cells
daughter cells differ from parent cell/ produces genetic variation
produces gametes for reproduction
chromosome number is halved/1n/haploid
homologous chromosomes join together/ pair
exchange of material between chromosomes/ crossing over

36
Q

What are chromosomes like in prokaryotes?

A
  • prokaryotes have one chromosome- made of a circular DNA molecule
  • some prokaryotes contain plasmids but eukaryotes do not have plasmids
  • plasmids are small circular pieces of DNA molecules that are not connected to the main bacterial chromosome
  • plasmids replicate independently from the main bacterial chromosome
  • plasmids are useful in genetic modification techniques
37
Q

What are chromosomes like in eukaryotes?

A
  • when the cell is not dividing, the chromosomes are not visible. The genetic material is said to be in the form of chromatin
  • chromatin is made of strands of DNA+ histone proteins
  • Chromosomes can be passed on to offspring
  • DNA is negatively charged
  • Histone proteins are positively charged
  • Histone proteins allow DNA to wrap itself around it
  • Histones allow supercoiling to occur- this allows DNA to become condensed
  • By DNA wrapping itself around histones, it stops transcription enzymes from accessing genes → this allows only certain areas of DNA to be involved in protein synthesis
  • Eukaryotes have more than one chromosome and each will contain a different set of instructions for the cell
38
Q

What are homologous chromosomes?

A
  • In the human cell→, 46 chromosomes can be put into 23 pairs of chromosomes = homologous chromosomes
  • The homologous chromosomes have a similar size and shape and therefore have similar genes
  • You have two chromosomes as one comes from your mother and one comes from your father
  • Homologous chromosomes may have the same genes, but they may not be identical→ the alleles from each parent may be different
39
Q

What are diploid and haploid cells?

A
  • most human body cells contain 46 chromosomes → these are diploid cells
  • sex cells contain half the number of chromosomes → these are haploid cells
  • normal body cell = 2n (diploid)
  • gamete (sex cell)= n (haploid)
40
Q

What is a kayrogram?

A

Karyogram- a representation of the chromosomes found in a cell arranged in order of ascending shape and size

41
Q

How do karyograms let us karyotype?

A
  • The shape is determined by the position of the centromere
  • Karyotype also includes their length, banding patterns and centromere position
  • You can use different staining techniques to show different banding patterns and centromere position
  • Light and dark bands show the position of different genes- the size and location of the bands on chromosomes make each chromosome unique
  • The position of the centromeres can vary too
42
Q

How does sex determination work in humans?

A
  • the sex chromosomes are the 23rd pair of chromosomes
  • they determine the gender of an individual
  • the X chromosome is longer than the Y chromosome
  • This means that the X chromosome contains more genes
  • All other 22 pairs of chromosomes (autosomal chromosomes) are the same size and shape
  • In humans, there is a 50% chance of having a male child
  • When a trait is more common with one sex than the other → likely to be sex-linked + the location of the gene is likely to be on the X -X-chromosome or Y- Y-chromosome
  • If there is no pattern to the frequency of a trait between sexes → it is likely to be an autosomal trait
43
Q

What is autoradiography and why is it used?

A
  • Autoradiography- a technique that uses radiation from a substance that is then captured on a photographic film or camera sensor
  • The images formed by autoradiography are exposed to radioactive particles released by the substance itself
  • DNA will give out radioactive particles + the images produced can be used to measure the length of DNA strands
44
Q

What is the John Cairns technique and how does it work? (autoradiography)

A
    • Injecting radioactive materials (called radio markers) into the DNA samples to expose the film faster
    • Isotopes can be used
    • These are radioactive and can be used as radio markers
    • The radio markers can be tracked so one can follow how DNA replicates
    • Using this technique Cairns in 1962 demonstrated that a bacterium’ chromosome is made up of a single circle of DNA + it replicates by being unzipped
45
Q

What is non-disjunction?

A
  • Non- disjunction refers to the chromosome failing to separate correctly, resulting in gametes with one extra, or one missing chromosome (aneuploidy)
  • The failure of chromosomes to separate may occur via:
    • Failure of homologues to separate in Anaphase I (resulting in four affected daughter cells)
    • Failure of sister chromatids to separate in anaphase II (resulting in only two daughter cells being affected)
  • it can lead to Down syndrome
46
Q

what are chromosomal abnormalities?

A
  • If a zygote is formed from a gamete that has experienced a non-disjunction event, the resulting offspring will have extra or missing chromosomes in every cell of their body
47
Q

What is Down syndrome?

A
  • Individuals with Down syndrome have three copies of chromosome 21 (trisomy 21)
    • one of the parental gametes had two copies of chromosome 21 as a result of non-disjunction
    • the other parental gamete was normal and had a single copy of chromosome 21
    • when two gametes fused during fertilisation, the resulting zygote had three copies of chromosome 21
48
Q

What is a gene?

A

A gene is a heritable factor that consists of a length of DNA and influences a specific characteristic

  • each chromosome carries thousands of genes
  • genes are formed using bases
  • genome= the complete set of an organism base sequence
49
Q

What is the position where a gene is located called?

A

Locus

50
Q

What are alleles and how were they discovered?

A
  • Gregor Mendel pea plant experiments showed that there were characteristics that were inherited by offspring + there were different forms of characteristics
  • Nowadays we know these are genes + you can get different forms of a gene called alleles
  • Alleles occupy the same position on chromosomes; they have the same locus
51
Q

How do alleles differ?

A

Alleles differ from each other by one or a few bases only

  • genes are made of sequences of bases
  • they can be hundreds of thousands of bases in length
  • there may only be one base or a few bases that would change in a gene to form an allele
52
Q

How are new alleles formed?

A

New Alleles are formed by mutation

  • Gene Mutations→ random change in the DNA base sequence of a gene
  • The most common is base substitution→ one base is replaced by another
  • Mutations may be beneficial, harmful or neutral to the survival chances of an organism
  • Mutations in somatic cells will be eliminated once the organism dies
  • Mutations in sex cells can be inherited by offspring and cause a genetic disease
53
Q

What leads to sickle cell anaemia?

A
  • Mutation in the gene that codes for the alpha-globin polypeptide in haemoglobin
  • The symbol for the gene is Hb
  • Most humans have Hb^A
  • In sickle cell anaemic individuals- the sixth codon in the alpha-globin gene changes from GAG to GTG
  • This change forms the Hb^S form of the polypeptide
  • This mutation is only inherited if it occurs within a sex cell
  • During protein synthesis (transcription phase)- due to the mutation the 6th amino acid will be valine instead of glutamic acid
  • This change causes the haemoglobin molecules to become sticky in low oxygen concentrations
  • This causes the haemoglobin to become rigid and causes the haemoglobin molecule to form bundles within a red blood cell
  • This causes the red blood cells to form sickle shapes
  • Sickle cells can become trapped in capillaries→ reducing blood flow to tissues
  • The bundles of haemoglobin in red blood cells break up in high oxygen concentrations and the cells can return to their normal shape
  • When this occurs it damages the plasma membranes of the red blood cells and shortens the lifespan of the red blood cells (4 days), when they are normally 60-90 days
  • The body cannot replace the red blood cells fast enough so anaemia develops.
54
Q

What is the human genome project?

A
  • In 1990- an international project to sequence the complete human genome
  • Scientists catalogued all base sequences
  • In 2003- the sequencing was complete
  • Now scientists are working on understanding which sequences are genes and what they code for.
  • Before the mapping, only 100 loci for genetic diseases were known- after mapping was completed over 1400 were known- now the number has increased rapidly
  • It allows scientists to compare genetic makeups from across the world and give a detailed insight into our past and how humans have changed over time
55
Q

How can medicines be made using DNA?

A
  • The information from the Human Genome Project has helped with the production of new medicines
  • The steps involved
    • identifying natural beneficial molecules in healthy individuals
    • identifying the gene that codes for the beneficial molecule of interest
    • make copies of the code to make the desired molecule in labs
    • distribute the therapeutic protein as a new treatment
56
Q

What are the consequences of mutations in genes that control the cell cycle?

A
  • Tumours are abnormal cell growths resulting from uncontrolled cell division and can occur in any tissue or organ
    • diseases caused by the growth of tumours are collectively known as cancers
57
Q

What is a mutagen?

A
  • A mutagen is an agent that changes the genetic material of an organism (either acts on the DNA or the replicative machinery)
  • Mutagens can be physical, chemical or biological in origin:
    • Physical- sources of radiation including X-rays (ionising), UV light and radioactive decay
    • Chemical- DNA interacting substances including reactive oxygen species and metals
    • Biological- Viruses, certain bacteria and mobile genetic elements (transposons)
  • Mutagens that lead to the formation of cancer are further classified as carcinogens
58
Q

What are oncogenes and how do they relate to cancer?

A
  • An oncogene is a gene that has the potential to cause cancer
  • Most cancers are caused by mutations to two basic classes of genes- proto-oncogenes and tumour suppressor genes
    • Proto-oncogenes code for proteins that stimulate the cell cycle and promote cell growth and proliferation
    • Tumour supressor genes code for proteins that repress the cell cycle progression and promote apoptosis ← programmed cell death
  • When a proto-oncogene is mutated or subjected to increased expression it becomes a cancer-causing oncogene
  • Tumour suppressor genes are sometimes referred to as anti-oncogenes as their normal function prevents cancer
59
Q

What is metastasis (cancer)

A

Metastasis is the spread of cancer from one location (primary tumour) to another, forming a secondary tumour

60
Q

What are the types of tumour?

A

Tumour cells may either remain in their original location (benign) or spread and invade neighbouring tissue (malignant)

61
Q

What is a benign tumour?

A
  • A tumour that tends to grow slowly and does not spread to other parts of the body
  • They absorb nutrients and enlarge and may compress surrounding tissues
  • They do not spread from its site of initiation
62
Q

What is a malignant tumour?

A
    • A tumour that grows rapidly, invades and destroys normal tissues and spreads throughout the body
    • consists of cells that secrete signals triggering growth of blood and lymph vessels to serve tumour cells at the expense of other tissues
    • Attachments to other cells are lost and then malignant cells may be carried around the body, setting up secondary growths
    • The functions of invaded organs are typically impaired or obstructed
63
Q

What is a primary tumour?

A

A cancer growing at the site where the abnormal growth first occured

64
Q

What is a secondary tumour?

A

Formed when cancerous cells detach from the primary tumour, penetrate the walls of lymph or blood vessels, and circulate around the body, causing tumours elsewhere

Biology HL (8 decks)

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