AQA A Level biology Mitotic cell cycle summary notes
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Course
9700 As biology (9700)
Institution
Cambridge College
These notes are for AQA A Levek biology topic mitotic cell cycle and cover details of,
1. Interphase: The period between two successive cell divisions, during which the cell grows and replicates its DNA.
2. Prophase: The first stage of mitosis, during which the chromatin condenses into visible ch...
A level notes series by Emaan Atif
Mitotic cell cycle
Describe the structure of a chromosome;
Chromosomes are made of one very long, condensed DNA molecule associated with proteins (in
eukaryotic cells)
DNA:
1. Deoxyribonucleic acid, is the inherited material.
2. Controls a variety of visible characteristics and invisible characteristics. Visible characteristics
include race, coloring and sex. An invisible characteristic includes blood groups and susceptibility
to inherited diseases.
3. Made up of molecules called nucleotides. Each nucleotide contains a sugar group, a phosphate
group, and a nitrogen base.
4. The four types of nitrogen bases are adenine (A), thymine (T), guanine (G) and cytosine (C).
Adenine and thymine are paired by two H bonds, whereas cytosine and guanine are paired by
three H bonds.
5. DNA has a double helical structure.
6. The outer edges are formed by alternating deoxyribose sugar molecules and phosphate groups,
which make up the sugar-phosphate backbone.
7. The two strands run in opposite directions. This means the nucleotides in each strand of DNA are
exactly complementary to that in the other strand. The nitrogenous bases are positioned inside
the helix structure. The bases, located inside the double helix, are stacked. The twists or turns in
the two sugar-phosphate backbones of the double helix occurs every ten base pairs, which
maximizes efficiency of the base-pair packing.
8. The chromosome is made of DNA containing many genes. Every gene comprises of the
particular set of instructions for a particular function or protein-coding. Speaking in usual terms,
genes are responsible for heredity.
9. DNA Packaging: Double-helix DNA is associated with histone proteins DNA wraps and coils
around histone proteins, Forms chromatin. Usually During cell division, DNA undergoes
further coiling, Form tightly-coiled chromosomes.
, Genes:
1. 1 chromosome has several thousand genes
2. Genes = DNA that codes for protein
3. Locus (plural: loci) = Position of gene in a chromosome
4. If same species, genes are on same chromosome and loci But can be different alleles.
5. Alleles = Different-forms of one gene. Can be dominant or recessive, dominant always
shows itself.
Histone proteins:
1. Histones are basic proteins found in chromosomes. They help in packing and organization
of DNA helix in chromatin fiber in the nucleus. They also regulate gene expression.
2. In eukaryotes, DNA is highly condensed and is wrapped around histones to form chromatin
thread, which further condenses to form chromosomes. Histones provide a structural
framework to chromosomes and make it more compact to fit inside the nucleus.
3. Histones are positively charged proteins that help in the coiling of DNA.
4. They help in DNA regulation as well.
5. The basic amino acids that make up the histone proteins are Arginine and Lysine.
6. Arginine is a hydrophilic (water-loving) basic amino acid.
7. Lysine is the most basic amino acid.
8. They are basic as they have a positive charge on their side chains at the physiological pH.
Homologous chromosomes and sister chromatids:
1. Pairs of chromosomes found in diploid cells
2. 1 maternal and 1 paternal
3. Similar centromere‟s position
4. Similar chromosome‟s size and shape
5. Same genes, different alleles, may have same alleles.
6. During interphase (S phase) the DNA replicates to create two identical strands of DNA
called chromatids, joined together by a narrow region called the centromere
7. The two chromatids that make up the double structure of a chromosome are known
as „sister chromatids‟
8. It is important that the sister chromatids are identical (contain the same genes)
because this is key to cell division, as one chromatid goes into one daughter cell and
one goes into the other daughter cell during mitosis, ensuring the daughter cells
are genetically identical
9. Each chromatid is made up of one very long, condensed DNA molecule, which is made
up of a series of genes
10. The ends of the chromatids in chromosomes are „sealed‟ with protective
structures called telomeres.
, Importance of mitosis in living processes:
1. Genetic stability- Mitosis helps in the splitting of chromosomes during cell division and generates
two new daughter cells. Therefore the chromosomes form from the parent chromosomes by
copying the exact DNA. Therefore, the daughter cells formed as genetically uniform and identical to
the parent as well as to each other. Thus mitosis helps in preserving and maintaining the genetic
stability of a particular population.
2. Growth- Mitosis help in increasing the number of cells in a living organism thereby playing a
significant role in the growth of a living organism.
3. Replacement and regeneration of new cells- Regeneration and replacement of worn-out and
damaged tissues is a very important function of mitosis in living organisms. Mitosis helps in the
production of identical copies of cells and thus helps in repairing the damaged tissue or replacing
the worn-out cells. But the degree of regeneration and replacement in multicellular organisms vary
from one another. For example, mitosis process is used in order to regrowth the legs of newts and
crustaceans. However, the degree of regrowth may vary.
4. Asexual reproduction- Mitosis is used in the production of genetically similar offspring. For
example budding of hydra and yeast, binary fission in amoeba, etc.
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