25 terms
All cells arise from other cells
In a multicellular organism, most cells lose the ability to divide once they specialise. Only certain cell types keep that ability throughout life.
Cells
All cells arise from other cells
Some eukaryotic cells — cells that contain a nucleus — repeatedly follow a fixed sequence of stages called the cell cycle. Each cycle ends with the cell dividing to produce new cells.
Cells
All cells arise from other cells
Before a cell divides, it must copy all of its DNA. This copying happens during interphase — the preparation stage of the cell cycle.
Cells
All cells arise from other cells
Mitosis is a type of cell division. It splits one cell into two new cells, and each new cell carries an exact copy of the original cell's DNA.
Cells
All cells arise from other cells
Mitosis splits one cell into two identical cells. Chromosomes go through five named stages as this happens. Each stage has a distinct, testable behaviour.
Cells
All cells arise from other cells
During cell division, protein fibres called spindle fibres attach to chromosomes and pull the two identical copies apart. Each copy moves to opposite ends of the cell.
Cells
All cells arise from other cells
After a cell copies and separates its DNA, cytokinesis splits the cytoplasm to produce two new daughter cells. The word 'usually' matters — the process can occasionally fail.
Cells
All cells arise from other cells
Normally, cells divide in a tightly controlled way. When that control breaks down, cells divide without stopping — forming a tumour, which can become cancer.
Cells
All cells arise from other cells
Cancer cells divide too fast and without control. Many treatments work by targeting specific stages of cell division to slow or stop this process.
Cells
All cells arise from other cells
Bacteria divide by splitting in two — a process called binary fission. The cell copies its DNA first, then splits its cytoplasm to make two new daughter cells.
Cells
All cells arise from other cells
Viruses are not alive, so they cannot divide like cells do. Instead, a virus injects its genetic material into a host cell and forces that cell to make new virus copies.
Cells
All cells arise from other cells
Meiosis is a different type of cell division from mitosis. AQA covers it separately in section 3.4.3, not here.
Cells
All cells arise from other cells
New cells can only come from pre-existing cells, and in eukaryotes (organisms whose cells have a nucleus) this happens through a tightly regulated sequence of events called the cell cycle, which culminates in mitosis — a type of cell division that produces two genetically identical daughter cells. Prokaryotes (cells without a nucleus, such as bacteria) divide by a simpler process called binary fission, while viruses bypass cell division entirely by hijacking a host cell's machinery to copy themselves. Understanding how cell division is controlled matters beyond the basics: when that control breaks down, cells can divide uncontrollably, leading to tumours and cancer — which is why many cancer treatments specifically target the cell cycle.
Cells
Alteration of the sequence of bases in DNA can alter the structure of proteins
A gene mutation is a change to the base sequence of a DNA molecule. Several types exist, and they most often arise when DNA copies itself before cell division.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
Gene mutations can happen by chance at any time. Certain environmental factors, called mutagenic agents, make mutations happen more often.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
A mutation changes the base sequence of a gene. Because bases code for amino acids, a changed sequence can produce a different chain of amino acids — a different polypeptide.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
Each amino acid in a protein is coded for by a three-base sequence called a codon (or triplet). A substitution mutation swaps one base for another, producing a new codon. The genetic code is described as degenerate, meaning most amino acids are coded for by more than one codon. Because of this redundancy, a substitution that changes the third base of a codon very often still codes for the same ami
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
Adding or deleting a base shifts every codon after that point. This scrambles the amino acid sequence from that position onwards.
The control of gene expression
Alteration of the sequence of bases in DNA can alter the structure of proteins
A gene mutation is a change to the sequence of bases in DNA, and these changes can arise spontaneously during DNA replication or be triggered by mutagenic agents — environmental factors such as UV radiation or certain chemicals that increase the rate at which mutations occur. Because the base sequence of a gene codes for the order of amino acids in a protein, some mutations alter the resulting polypeptide's structure and potentially its function. However, the impact of a mutation depends on its type: a substitution (one base swapped for another) may have no effect due to the degenerate genetic code, whereas an insertion or deletion can cause a frame shift that disrupts every codon downstream, with far-reaching consequences for the protein produced.
The control of gene expression
ATP
ATP is the molecule cells use to carry and transfer energy. It is built from three parts: a sugar called ribose, a base called adenine, and three phosphate groups.
Biological molecules
ATP
An enzyme called ATP hydrolase breaks ATP apart using water. It splits ATP into ADP and a free phosphate group, releasing energy the cell can use.
Biological molecules
ATP
When ATP breaks down, it releases energy. Cells link this release directly to processes that need energy, such as muscle contraction or building proteins.
Biological molecules
ATP
When ATP breaks down, it releases a phosphate group. That phosphate can attach to other molecules, making them more chemically reactive and easier for cells to use.
Biological molecules
ATP
Cells rebuild ATP by joining ADP and inorganic phosphate (Pi) together. The enzyme ATP synthase catalyses this reaction during photosynthesis and respiration.
Biological molecules
ATP
Adenosine triphosphate (ATP) is the universal energy currency of cells — a small nucleotide derivative built from ribose, adenine and three phosphate groups, whose structure allows it to store and release energy on demand. When a cell needs energy, the enzyme ATP hydrolase breaks one phosphate bond in a hydrolysis reaction (splitting using water), releasing inorganic phosphate (Pi) and converting ATP into ADP; this released energy can directly power energy-requiring processes such as muscle contraction, active transport and biosynthesis. Understanding ATP is essential because it bridges the energy-releasing reactions of respiration and photosynthesis — where ATP synthase rebuilds ATP from ADP and Pi — with virtually every energy-consuming process in a living organism.
Biological molecules