Biology Notes:
,1.5 Notes:
Cells can only be formed by division of pre-existing cells.
Spontaneous generation and the origin of cells: Spontaneous generation is the formation of
living organisms from non-living matter ex. Greek philosopher saying plant Silphium grew
from soil where it had not been before. Scientists tried to prove this:
1. Francesco Redi: left meat out to rot. When meat was uncovered flies came in contact with
it so maggots were developed. When covered, no maggots were found.
2. Lazzaro Spallanzani: boiled 8 containers of soup, only the 4 left open showed organisms
growing in them.
3. Louis Pasteur: made nutrient broth from yeast and sugar. When kept in sealed flask, no
fungi or organisms appeared. But when air was passed through cotton wool in a tube (filtering
out microscopic particles in air) and that wool was placed in a sealed jar, after 36 hours
microorganisms and mold in broth.
Swan-neck experiment: placed broth in flasks with long necks, melted glass of necks and (bent
it into several shapes). Some broth boiled (kill organisms) and some left as control. Fungi and
organisms appeared only in unboiled flasks. When he snapped some of the necks of the flasks,
organisms were soon apparent and decomposed broth. 1860, he concluded that the swan
necks prevented organisms from air entering & no organisms appeared spontaneously. Air
could enter through the swan-neck shape of the flasks, but microbes could not.
Other evidence: 1. Cell is a complex structure and cannot be produced from simpler subunits.
2. No example of increase of number of cells in population/organism/tissue without cell
division. 3. Viruses produced from simpler subunits but are not cells and only produced inside
host cell they infected.
Origin of the first cells: Must have arisen from non-living material, living cells may have
evolved over hundreds of millions of years.
1. Production of Carbon Compounds (e.g. sugars & amino acids)
→ Miller-Urey experiment
→ Steam passed through methane, hydrogen & ammonia mixture
→ Mixture simulated early atmospheric conditions on Earth
→ Electrical discharges used to simulate lightning
→ Amino Acids & carbon compounds produced! – needed for life
2. Assembly of Carbon Compounds into Polymers
→ Deep sea vents at cracks in the Earth’s surface release hot water (possible site of
first carbon compounds)
→ this water is carrying inorganic chemicals like iron sulphide
→ These represent supplies of energy for the assembly of these carbon compounds
into polymers
3. Formation of Membranes
→ If amphipathic carbon compounds (e.g. phospholipids) were found among the
first carbon compounds they would naturally assemble into bilayers
→ Experiments show these bilayers form vesicles, resembling a cell membrane. This
, would have allowed different internal chemistry from that of the surroundings to
develop
4. Development of a Mechanism for Inheritance
→ Living organisms have genes made of DNA
→ to replicate DNA enzymes are needed as catalysts
→ To synthesise enzymes DNA (genes) is needed! -> how did it start?
→ Solution = RNA was the early genetic material, not DNA
→ RNA stores information in the same way as DNA, but crucially is self-replicating
and can itself act as a catalyst
Endosymbiosis and eukaryotic cells:
Endosymbiotic theory: 1. mitochondria were once free-living prokaryotic organisms with
aerobic respiration. Larger prokaryotes that could only respire anaerobically engulfed them
by endocytosis, allowed them to live inside their cytoplasm -> internal bacteria passed on
through generations and over hundreds of millions of years became mitochondria. The two
prokaryotes had a symbiotic relationship, so both benefited (mutualistic): small was supplied
with food, large was supplied with energy from aerobic respiration. Natural selection favored
cells with endosymbiotic relationship. 2. Origin of chloroplasts: prokaryote that had
developed photosynthesis was taken in by a larger one and allowed to divide, it could have
become chloroplasts – endosymbiotic relationship. – Development of nucleus: A prokaryote
grows in size and develops folds in its membrane to maintain an efficient surface area to
volume ratio. The infoldings are pinched off forming an internal membrane. The nucleoid
region is enclosed in the internal membrane and hence becomes the nucleus
Chloroplasts and mitochondria have features suggesting they evolved from independent
prokaryotes:
- Have their own genes on a circular DNA molecule like that of prokaryotes
- Have their own 70S ribosomes of size and shape typical of prokaryotes
- Transcribe DNA and use mRNA to synthesize some of their own proteins
- Can only be produced by division of pre-existing mitochondria and chloroplasts
- Reproduce in a similar process to binary fission
- Have double membranes
, 1.6 Notes: PPT good for cell cycle explanation, cytokinesis differences, slide 19 + 25
Mitosis: the division of the nucleus of a eukaryotic cell into two genetically identical daughter
nuclei, allowing the cell to divide into two daughter cells. Before mitosis, DNA in nucleus is
replicated in interphase. Each chromosome converted from single DNA molecule to 2
identical DNA molecules (chromatids), one of which passed to daughter nucleus in mitosis.
Mitosis occurs during embryonic development, growth, tissue repair, asexual reproduction.
Phases: Prophase, metaphase, anaphase, telophase.
Interphase: active phase of the cell cycle with many processes occurring in nucleus and
cytoplasm (between one cell division and next) – metabolic processes. DNA replication in
nucleus and protein synthesis in cytoplasm only occur during interphase. Number of
mitochondria in cytoplasm increase, in plant cells and algae numbers of chloroplasts increase
and synthesize cellulose. 70% of cell cycle.
Phases: 1. G1 phase: prepare for synthesis - cellular contents apart from chromosomes are
duplicated (some do not go past this and enter a phase called G0 which may be permanent).
2. S phase: genetic material in nucleus (chromosomes) duplicated so both new cells have
complete set of genes. 3. G2 phase: organelles of the cell duplicate and cell is prepared to
enter mitosis.
Supercoiling of chromosomes: chromosomes condense by supercoiling during mitosis. During
mitosis, 2 chromatids making up a chromosome must be separated and moved to opposite
poles of cell, but DNA molecules in the chromosomes are very long: human nuclei
<5micrometers, but DNA molecules inside >50000micrometers -> chromosomes must be
condensed to shorter structurers (during first stage of mitosis). Condensation occurs by
supercoiling: repeatedly coiling DNA molecule to make chromosome shorter and wider.
Histones associated with proteins (nucleosome) help supercoiling, also enzymes.
Phases of Mitosis:
1. Prophase: Early: chromosomes become shorter and wider by supercoiling. Nucleolus
breaks down. Microtubule organizing centers (MTOC) grow microtubules to link poles of cell
-> mitotic spindle. Late: Nuclear membrane breaks down, spindle grows more.
2. Metaphase: Microtubules growing attach to centromeres on each chromosome. The
chromatids of chromosome attach to microtubules of different poles. The chromosomes line
up on the metaphase plate if attachment is correct.
3. Anaphase: Each centromere divides so pairs of sister chromatids separate. Spindle
microtubules pull them towards the poles of the cell, so mitosis produces 2 genetically
identical nuclei (ensured by the way spindle microtubules were attached in metaphase).
4. Telophase: Chromatids reached poles and are now called chromosomes. Chromosomes are
pulled into a tight group near the MTOC, a nuclear membrane forms around them.
Chromosomes uncoil and nucleolus is formed. The cell is usually already dividing and the 2
daughter cells enter interphase.
The mitotic index: is the ratio between the number of cells in mitosis in a tissue and the total
number of cells observed in the tissue.
Mitotic index = number of cells in mitosis / total number of cells -> UNCLEAR ON PHOTOS
,1.5 Notes:
Cells can only be formed by division of pre-existing cells.
Spontaneous generation and the origin of cells: Spontaneous generation is the formation of
living organisms from non-living matter ex. Greek philosopher saying plant Silphium grew
from soil where it had not been before. Scientists tried to prove this:
1. Francesco Redi: left meat out to rot. When meat was uncovered flies came in contact with
it so maggots were developed. When covered, no maggots were found.
2. Lazzaro Spallanzani: boiled 8 containers of soup, only the 4 left open showed organisms
growing in them.
3. Louis Pasteur: made nutrient broth from yeast and sugar. When kept in sealed flask, no
fungi or organisms appeared. But when air was passed through cotton wool in a tube (filtering
out microscopic particles in air) and that wool was placed in a sealed jar, after 36 hours
microorganisms and mold in broth.
Swan-neck experiment: placed broth in flasks with long necks, melted glass of necks and (bent
it into several shapes). Some broth boiled (kill organisms) and some left as control. Fungi and
organisms appeared only in unboiled flasks. When he snapped some of the necks of the flasks,
organisms were soon apparent and decomposed broth. 1860, he concluded that the swan
necks prevented organisms from air entering & no organisms appeared spontaneously. Air
could enter through the swan-neck shape of the flasks, but microbes could not.
Other evidence: 1. Cell is a complex structure and cannot be produced from simpler subunits.
2. No example of increase of number of cells in population/organism/tissue without cell
division. 3. Viruses produced from simpler subunits but are not cells and only produced inside
host cell they infected.
Origin of the first cells: Must have arisen from non-living material, living cells may have
evolved over hundreds of millions of years.
1. Production of Carbon Compounds (e.g. sugars & amino acids)
→ Miller-Urey experiment
→ Steam passed through methane, hydrogen & ammonia mixture
→ Mixture simulated early atmospheric conditions on Earth
→ Electrical discharges used to simulate lightning
→ Amino Acids & carbon compounds produced! – needed for life
2. Assembly of Carbon Compounds into Polymers
→ Deep sea vents at cracks in the Earth’s surface release hot water (possible site of
first carbon compounds)
→ this water is carrying inorganic chemicals like iron sulphide
→ These represent supplies of energy for the assembly of these carbon compounds
into polymers
3. Formation of Membranes
→ If amphipathic carbon compounds (e.g. phospholipids) were found among the
first carbon compounds they would naturally assemble into bilayers
→ Experiments show these bilayers form vesicles, resembling a cell membrane. This
, would have allowed different internal chemistry from that of the surroundings to
develop
4. Development of a Mechanism for Inheritance
→ Living organisms have genes made of DNA
→ to replicate DNA enzymes are needed as catalysts
→ To synthesise enzymes DNA (genes) is needed! -> how did it start?
→ Solution = RNA was the early genetic material, not DNA
→ RNA stores information in the same way as DNA, but crucially is self-replicating
and can itself act as a catalyst
Endosymbiosis and eukaryotic cells:
Endosymbiotic theory: 1. mitochondria were once free-living prokaryotic organisms with
aerobic respiration. Larger prokaryotes that could only respire anaerobically engulfed them
by endocytosis, allowed them to live inside their cytoplasm -> internal bacteria passed on
through generations and over hundreds of millions of years became mitochondria. The two
prokaryotes had a symbiotic relationship, so both benefited (mutualistic): small was supplied
with food, large was supplied with energy from aerobic respiration. Natural selection favored
cells with endosymbiotic relationship. 2. Origin of chloroplasts: prokaryote that had
developed photosynthesis was taken in by a larger one and allowed to divide, it could have
become chloroplasts – endosymbiotic relationship. – Development of nucleus: A prokaryote
grows in size and develops folds in its membrane to maintain an efficient surface area to
volume ratio. The infoldings are pinched off forming an internal membrane. The nucleoid
region is enclosed in the internal membrane and hence becomes the nucleus
Chloroplasts and mitochondria have features suggesting they evolved from independent
prokaryotes:
- Have their own genes on a circular DNA molecule like that of prokaryotes
- Have their own 70S ribosomes of size and shape typical of prokaryotes
- Transcribe DNA and use mRNA to synthesize some of their own proteins
- Can only be produced by division of pre-existing mitochondria and chloroplasts
- Reproduce in a similar process to binary fission
- Have double membranes
, 1.6 Notes: PPT good for cell cycle explanation, cytokinesis differences, slide 19 + 25
Mitosis: the division of the nucleus of a eukaryotic cell into two genetically identical daughter
nuclei, allowing the cell to divide into two daughter cells. Before mitosis, DNA in nucleus is
replicated in interphase. Each chromosome converted from single DNA molecule to 2
identical DNA molecules (chromatids), one of which passed to daughter nucleus in mitosis.
Mitosis occurs during embryonic development, growth, tissue repair, asexual reproduction.
Phases: Prophase, metaphase, anaphase, telophase.
Interphase: active phase of the cell cycle with many processes occurring in nucleus and
cytoplasm (between one cell division and next) – metabolic processes. DNA replication in
nucleus and protein synthesis in cytoplasm only occur during interphase. Number of
mitochondria in cytoplasm increase, in plant cells and algae numbers of chloroplasts increase
and synthesize cellulose. 70% of cell cycle.
Phases: 1. G1 phase: prepare for synthesis - cellular contents apart from chromosomes are
duplicated (some do not go past this and enter a phase called G0 which may be permanent).
2. S phase: genetic material in nucleus (chromosomes) duplicated so both new cells have
complete set of genes. 3. G2 phase: organelles of the cell duplicate and cell is prepared to
enter mitosis.
Supercoiling of chromosomes: chromosomes condense by supercoiling during mitosis. During
mitosis, 2 chromatids making up a chromosome must be separated and moved to opposite
poles of cell, but DNA molecules in the chromosomes are very long: human nuclei
<5micrometers, but DNA molecules inside >50000micrometers -> chromosomes must be
condensed to shorter structurers (during first stage of mitosis). Condensation occurs by
supercoiling: repeatedly coiling DNA molecule to make chromosome shorter and wider.
Histones associated with proteins (nucleosome) help supercoiling, also enzymes.
Phases of Mitosis:
1. Prophase: Early: chromosomes become shorter and wider by supercoiling. Nucleolus
breaks down. Microtubule organizing centers (MTOC) grow microtubules to link poles of cell
-> mitotic spindle. Late: Nuclear membrane breaks down, spindle grows more.
2. Metaphase: Microtubules growing attach to centromeres on each chromosome. The
chromatids of chromosome attach to microtubules of different poles. The chromosomes line
up on the metaphase plate if attachment is correct.
3. Anaphase: Each centromere divides so pairs of sister chromatids separate. Spindle
microtubules pull them towards the poles of the cell, so mitosis produces 2 genetically
identical nuclei (ensured by the way spindle microtubules were attached in metaphase).
4. Telophase: Chromatids reached poles and are now called chromosomes. Chromosomes are
pulled into a tight group near the MTOC, a nuclear membrane forms around them.
Chromosomes uncoil and nucleolus is formed. The cell is usually already dividing and the 2
daughter cells enter interphase.
The mitotic index: is the ratio between the number of cells in mitosis in a tissue and the total
number of cells observed in the tissue.
Mitotic index = number of cells in mitosis / total number of cells -> UNCLEAR ON PHOTOS
