Inheritance and variation Flashcards
Mitosis gives rise to cells that are
genetically identical
What is mitosis used for?
Growth, repair of damaged tissues, replacement of cells and asexual reproduction
Meiosis gives rise to cells that are
genetically different
Meiosis is used to produce
gametes (sex cells)
sexual reproduction
It is a process involving the fusion of two gametes (sex cells) to form a zygote (a fertilised egg cell) and the production of offspring that are genetically different from each other.
What are the gametes of animals?
sperm cells and egg cells
What are the gametes of flowering plants?
pollen cells and egg cells
Fertilisation
It is the fusion of the gamete nuclei, and as each gamete comes from a different parent,there is variation in the offspring
What process does the formation of gametes involve?
Meiosis
Does asexual reproduction involve sex cells or fertilisation?
No
Asexual reproduction
- Only one parent is required so there is no fusion of gametes and no mixing of genetic information
- As a result, the offspring are genetically identical to the parent and to each other (clones)
- Asexual reproduction is defined as a process resulting in genetically identical offspring from one parent
- Only mitosis is involved in asexual reproduction
Meiosis
-Cells in reproductive organs divide by meiosis to form gametes (sex cells)
-The number of chromosomes must be halved when the gametes are formed
-Otherwise, there would be double the number of chromosomes after they join at fertilisation in the zygote (fertilized egg)
-This halving occurs during meiosis, and so it is described as a reduction division in which the chromosome number is halved from diploid to haploid, resulting in genetically different cells
-It starts with chromosomes doubling themselves as in mitosis and lining up in the centre of the cell
-After this has happened the cells divide twice so that only one copy of each chromosome passes to each gamete
We describe gametes as being haploid – having half the normal number of chromosomes
Because of this double division, meiosis produces four haploid cells
Meiosis process
- Each chromosome has been duplicated, forming X-shaped chromosomes
- The nuclear membrane breaks down
- Maternal and paternal chromosomes pair up along the centre of the cell
- Sections of DNA are swapped between maternal and paternal chromosome pairs, creating recombinant chromosomes
- Chromosome pairs separate, moving to opposite sides of the cell
- First division: the chromosome pairs line up along the centre of the cell and are then pulled apart so that each new cell only has one copy of each chromosome
- Second division: the chromosomes line up along the centre of the cell and the arms of the chromosomes are pulled apart
- A total of four haploid daughter cells will be produced
What is the importance of meiosis?
- Produces gametes eg. sperm cells and egg cells in animals, pollen grains and ovum cells in plants
- Increases genetic variation of offspring
- Meiosis produces variation by forming new combinations of maternal and paternal chromosomes every time a gamete is made, meaning that when gametes fuse randomly at fertilisation, each offspring will be different from any others
Fertilisation
- Gametes join at fertilisation to restore the normal number of chromosomes
- When the male and female gametes fuse, they become a zygote (fertilised egg cell)
- This contains the full number of chromosomes, half of which came from the male gamete and half from the female gamete
- The zygote divides by mitosis to form two new cells, which then continue to divide and after a few days form an embryo
- Cell division continues and eventually many of the new cells produced become specialised (the cells differentiate) to perform particular functions and form all the body tissues of the offspring
- The process of cells becoming specialised is known as cell differentiation
What does each nucleotide consist of?
A sugar, phosphate and base
Advantages of sexual reproduction
- increases genetic variation
- the species can adapt to new environments due to variation, giving them a survival advantage
- disease is less likely to affect a population (due to variation)
Disadvantages of sexual reproduction
- takes time and energy to find mates
- difficult for isolated members of the species to reproduce
Adantages of asexual reproduction
- population can be increased rapidly when conditions are right
- can exploit suitable environments quickly
- more time and energy efficient
- reproduction is completed much faster than sexual reproduction
Disadvantages of asexual reproduction
- limited genetic variation in population- offspring are genetically identical to their parents
- population is vulnerable to changes in conditions and may only be suited for one habitat
- disease is likely to affect the whole population as there is no genetic variation.
Examples of plants which reproduce by both methods
- malarial parasites
- fungi
- plants
malarial parasites
- malaria is caused by parasites carried by mosquitoes
- The parasites are then transferred to a human when the mosquito feeds on the human’s blood
- These malarial parasites reproduce asexually in the human host, but sexually in the mosquito
Fungi
- Many fungi reproduce both asexually and sexually
- These species of fungi release spores, which develop into new fungi
- These spores can be produced via asexual or sexual reproduction
- Spores that are produced via sexual reproduction show variation (they are genetically different from each other)
Plants
- Many plants produce seeds via sexual reproduction but are also able to reproduce asexually
- They reproduce asexually in different ways:
- Some plants (eg. strawberry plants) produce ‘runners’ (stems that grow horizontally away from the parent plant, at the end of which new identical offspring plants form)
- Some plants (eg. daffodils) reproduce via bulb division (new bulbs form from the main bulb underground and then grow into new identical offspring plants)
Proteins
A sequence of three bases (triplet) codes for a specific amino acid = proteins (can be hormones, enzymes, collagen)
3 bases –> amino acid –> amino acid chain –> polypeptide–> protein
An amino acid molecule is made up of the elements
- carbon
- oxygen
- nitrogen
- hydrogen
Describe PRoTein synThesis (5)
1, The DNA, double helix strand opens (unwinds/unzips) by breaking the hydrogen bonds into single strands
2, The gene is copied onto the RNA strand (transcription)
3, mRNA leaves the nucleus, enters the cytoplasm and attatches itself to the ribosome
4, The ribosomes will ‘read’ the code for a specific amino acid. Amino acid chains will form a polypeptide (transLation)
5, Polypeptide will form the appropriate protein (growth and repair)
gene
a section of DNA which codes for a specific sequence of amino acids
allele
a form/version of a particular gene or mutation
genotype
The inherited alleles that will determine the phenotypee
phenotype
genotype + environment
- the physical expression of the genotype eg: eye colour, height
homozygous
a pair of identical alleles
heterozygous
a pair of non-identical alleles
dOminant
- only one allele needs to be present for the phenotype/genetic order to be expressed
recessive
- both alleles need to be present for the phenotype/ genetic order to be expressed
codominant
- both the alleles are responsible for the phenotype
Cells in reproductive organs divide by what?
meiosis to form gametes (sex cells)
When gametes are formed, the number of chromosomes must be
halved, or there will be double the number of chromosomes after they join at fertilisation in the zygote.
Deoxyribonucleic acid is the
molecule which contains instructions for growth and development of all organisms.
The genetic material in the nucleus of the cell is composed of a chemical called
DNA.
DNA is contained in structures called
chromosomes
DNA is
a polymer made up of two strands forming a double helix
Chromosomes are located in
the nucleus of the cells
What is a chromosome?
It is a long strand of DNA wrapped around proteins.
Genes control our
characteristics
DNA
A biological molecule (polynucleotide) that contains our genetic code
genome
the complete genetic makeup of an organism
How many genes do we have?
Roughly about 21000 genes
Describe protein synthesis
1, A protein is a string of amino acids. There are 20 amino acids in human protein. Each protein is coded by a single s
Mitosis
Produces genetically identical cells
meiosis
making eggs in ovaries or making sperm in testes
Trna transfers amino acids from
the cytoplasm to the ribosome
how are nucleotides joined?
Base pairing
