Cell Division & Inheritance
- Created by: Ashleigh Hockenhull
- Created on: 03-05-15 15:51
Human Body Cells
Body cells contain tow sets of 23 chromosomes arranged in pairs (46 in total)
Chromosomes contain genetic information
Gametes are sex cells. Gametes only have one set of 23 chromosomes
Fertilisation
During fertilisation, the female & male gametes fuse to produce a zygote - a single body cell with 23 new pairs of chromosomes. In each pair:
- one chromosome comes from the mother
- one chromosome comes from the father
The cell then dividies repeatedly by mitosis to form a new individual
Variation is caused due to the combinationof genes from the mother & father
Mitosis
Mitosis is the division of body cells to produce new cells. Mitosis occurs:
- for growth
- for repair
- in asexual reproduction (cells produced by asexual reproduction contain the same alleles as the parents)
During mitosis:
- a copy of each chromosome is made
- the cell then divides once to produce two new body cells
- the new cells contain exactly the same genetic information as the parent cell
Meiosis
Meiosis occurs in the testes & ovaries to produce the gametes (sperm and egg) for sexual reproduction
When a cell divides to form gametes:
- copies of the genetic material are made
- the cell then divides twice to form four gametes, each with a single set of chromosomes
Inheritance of the Sex Chromosome
Of the 23 pairs of chromosomes in the human body, one pair carries the genes that determines sex:
- in females, the sex chromosomes are identical and are called the X chromosomes
- in males, one sex chromosome (Y) is much shorter than the other (X)
As with all chromosomes, offspring inherit:
- one sex chromosome from the mother (always an X)
- one sex chromosome from the father (either an X or Y)
So, the sex of the offspring is decided by whether the ovum (egg) is fertilised by an X-carrying sperm or a Y-carrying sperm
Alleles
Some characteristics are controlled by a single gene. Genes may have different forms, or variations caled alleles. For examples:
- the gene that controls tongue-rolling ability has 2 alleles - either u can or u can't
- the gene that controls eye colour has two alleles - blue or brown
Sexual reproduction gives rise to variation because when gametes join during fertilisation:
- one allele for each gene comes from the mother
- one allele for each gene comes from the father
In a pair of chromosomes, the alleles for a gene can be the same or different. If they're different:
- one allele will be dominant
- one allele will be recessive
A dominant allele will always control the characteristic; it will express itself even if present on only one chromosome in a pair
A recessive allele will only control the characteristic if it is present on both chromosomes in a pair
Genetic Diagrams
In genetic diagrams you should use:
- capital letters for dominant alleles
- lower case letter for recessive alleles
When constructing genetic diagrams:
- clearly identify the alleles of the parents
- place each of these alleles in a seperate gamete
- join each gamete with the two gametes from the other parent
Following terms:
- genotype - combination of alleles that an individual has for a particular gene
- homozygous - an individual who carries two copies of the same allele for a particular gene
- phenotype - the expression of the genotype (characteristic shown)
Stem Cells
Most types of animal cells differentiate & become specialised at an early stage. Many plant cells retain the ability to differentiate throughout their life
Stem cells have the ability to develop into any kind of cell because they haven't yet differentiated. Stem cells in humans are found in:
- human embryos
- adult bone marrow
Human stem cells can be made to differentiate into any type of human cell. These cells could potentailly be used to help treat people with conditions like paralysis. There are some ethical issues surrounding the use of stem cells for scientific research:
- Against - some people believe that human embryos shouldn't be used bc they're a potential human life
- For - some people believe its more important to help people who are already living
Some embryo stem cells are taken from unwanted embryos from fertility clinics where they would be destroyed if not used for research. Some countries have banned stem cell research
The Structure of Chromosomes
Chromosomes are made up pf DNA. A DNA molecule consisits of two long strands that are coliled to form a double helix.
Each person has unique DNA (apart from identical twins) So, DNA can be used for identification. A gene is a small section of DNA.
Genes code for a particular characterisitc by providing a code for a combination of amino acids that make up a specific protein
Genetic Disorders
Some disorders are inherited, such as:
- polydactyly
- cystic fibrosis
People with polydactlyly have extra fingers or toes. Its caused by a dominant allele, so it can be passed on even if only one person has the disorder.
Cystic fibrosis is a disorder of the cell membranes. It's caused by a recessive allele, so both parents must carry the allele. Because the allele is recessive, the parents can be carriers without having the disorders themselves.
Embryos can be screened for the alleles that cause genetic disorders. This has enconomic, social & ethical issues.
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