meiosis and stuff Flashcards
What is meiosis?
Meiosis occurs when sex cells (gametes) are formed in the ovaries and testes.
what happens during meiosis
4 gametes are produced from one daughter cell
Each gamete has half the number of chromosomes as the parent cell (in humans, 23 chromosomes, not 23 pairs)
its important to produce gametes with half the chromosome number because Human body cells need 46 chromosomes. If each gamete contained 46, after fertilisation, body cells would contain 92 chromosomes (not viable). By halving the number in meiosis, after fertilisation each cell has the correct number of chromosomes
interphase
DNA duplicates - this means that each chromosone now consists of 2 sister chromatids
2 chromosones
1 chromatids each
prophase 1
Nuclear membrane disappears
homologus chromosones (which code for the same thing) pair up - Crossing over occurs - this meaans that non-sister chromatids exchange some genetic material CREATING VARIATION
Spindle fibres form
2 chromosones
2 chromatids each
metaphase 1
Homologous chromosome pairs line up along the equator of the cell.
Spindle fibres attach to the chromosomes at the centromere
2 chromosones
2 chromatids each
anaphase 1
2Homologous chromosome pairs are pulled apart to opposite poles – the 2 sister chromatids stay together.
Any swapped genetic material is pulled apart too. This creates genetic differenciation
2 chromosones
2 chromatids each
telophase 1 & cytokinesis
snuclear membranes form around the now divided genetic material.
After cytokinesis, 2 daughter cells are formed WITH HALF THE CHROMOSOME NUMBER OF THE PARENT CELL.
Each chromosome still consists of 2 chromatids, but the chromosome number is half of the original cell
1 chromosones
2 chromatids each
prophase 2
Nuclear membrane disappears
Spindle fibres form
1 chromosones
2 chromatids each
metaphase 2
chromosomes (still consisting of chromatids) line up along the equator.
Spindle fibres attach to the chromosome at the centromere
1 chromosones
2 chromatids each
anaphase 2
chromatids are pulled apart towards opposite poles by spindle fibres
1 chromosones
2 chromatids each
telophase 2 & cytokinesis
nuclear membranes form around the now divided genetic material
Post-cytokinesis four daughter cells are formed. Each is genetically different to one another, and different to the parent cell.
Each daughter cell also has half the chromosomes number of the parent cell – haploid
1 chromosones
1 chromatids each
Sexual reproduction
Sexual reproduction is where genetic information from two organisms (a father and a mother) is combined to produce offspring which are genetically different to either parent.
This involves the fusion of male and female gametes. Because there are two parents, the offspring contain a mixture of their parents’ genes and are genetically different to their parents.
Gametes
In sexual reproduction the mother and father produce gametes by meiosis - e.g. egg and sperm cells in animals. In humans, each gamete contains 23 chromosomes half the number of chromosomes in a normal cell. (Instead of having two of each chromosome, a gamete has just one of each.)
This is why the offspring inherits features from both parents - it’s received a mixture of chromosomes from its mother and its father (and it’s the chromosomes that decide how you turn out). This mixture of genetic information produces variation in the offspring.
Asexual reproduction
In asexual reproduction there’s only one parent so the offspring are genetically identical to that parent. Asexual reproduction happens by mitosis - an ordinary cell makes a new cell by dividing in two. The new cell has exactly the same genetic information as the parent cell - it’s called a clone.
Advantages of sexual reproduction
Produces variation - If the environment changes, variation gives a survival advantage by natural selection
disadvantages of sexual reproduction
Can be time consuming and energy inefficient
Advantages of asexual reproduction
There only needs to be one parent. This means that asexual reproduction uses less energy than sexual reproduction, because organisms don’t have to find a mate. This also means that asexual reproduction is faster than sexual reproduction.
Another advantage is that many identical offspring can be produced in favourable conditions.
disadvantages of asexual reproduction
If all offspring are identical, they could all be at risk, e.g. if a new disease develops
rerproduction with both sexual and asexual reproduction
fungi, malarial parasites and some plants can do both sexual and asexual reproduction.
Fungi
Fungi are made of masses of threads called hyphae (haploid)
In ideal conditions, fungi produce spores asexually. Spores disperse and germinate to produce clones of the parent
When conditions are not good, hyphae from 2 different fungi join to create a diploid hypha (sexual reproduction). The diploid hypha undergoes meiosis to produce haploid spores. These spores will be different to spores of either of the original parents
Plants e.g. strawberries
Sexually – gametes combine in pollination to form diploid seeds that germinate to produce a new plant. It will be genetically different to either parent
Asexually – can produce ‘runners’, that will create genetically identical plants. Can still produce new plants even is flowers are destroyed in frost, are eaten or are not pollinated.
Malarial parasite
Malaria is caused by a parasite that’s spread by mosquitoes. When a mosquito carrying the parasite bites a human, the parasite can be transferred to the human. The parasite reproduces sexually when it’s in the mosquito and asexually when it’s in the human host
Chromosomes
long Thread-like structures in the nucleus of a cell that contain DNA
DNA double helix
Shape of the DNA. 2 strands of nucleotides that wind up around each other like a twisted ladder to protect the bases.
DNA
DNA stands for deoxyribonucleic acid. It’s the chemical that all of the genetic material in a cell is made up from. It contains coded information - basically all the instructions to put an organism together and make it work. So it’s what’s in your DNA that determines what inherited characteristics you have and its sequence determines how our bodies are made.
Gene
Section of DNA found on a chromosone that contains the instructions for a particular characteristic.
DNA bases
Four chemicals found in DNA that make up the base sequence (A, T, C, G)
Genetic code
The sequence of bases within the DNA that ultimately codes for proteins
Structure of DNA - nucleotides
DNA is made up of a series of repeated units called nucleotides. Therefore, we call DNA a polymer
Structure of DNA
Each strand of DNA is made of a sugar-phosphate backbone (held together by phosphodiester bonds)
The two halves of the helix are held together by weak hydrogen bonds.
C-G = 3 H bonds
A-T = 2 H bonds
complementary base pairing
A and T always are complementary (they are always linked together on opposite strands)
C and G are complementary.
The bases consist of
A Adenine
T Thymine
G Guanine
C Cytosine
Genome
The Genome is the entire genetic material of an organism
benefits of nderstanding the human genome
Search for genes linked to different diseases
Better understand and possibly treat inherited disorders
Trace human migration patterns from prehistory - The human genome is mostly identical in all individuals, but as different populations of people migrated away from Africa, they gradually developed tiny differences in their genomes. By investigating these differences, scientists can work out when new populations split off in a different direction and what route they took.
When do Mutations occur?
Mutations occur continuously. They can occur spontaneously, e.g. when a chromosome isn’t quite replicated properly. However, the chance of mutation is increased by exposure to certain substances or some types of radiation.
Examples of mutations
Enzymes are proteins, if the shape of an enzyme’s active site is changed, its substrate may no longer be able to bind to it - see page 120.
Structural proteins like collagen could lose their strength if their shape is changed, making them pretty useless at providing structure and support.
silent mutations (in exons - coding pieces of DNA)
Not all mutations have a serious effect. We have two copies of every gene so if one is faulty, one may be okay. Sometimes if a base is changed in the DNA it does not code for a different amino acid/protein (silent mutation). If it is coding for an enzyme it may not lose its function.
mutations (in exons - coding pieces of DNA)
Some mutations change the DNA base sequence, and a different
amino acid is coded for, so we get a different protein. If this is an enzyme, it may mean it is not folded correctly (it would alter the intermolecular forces holding it in its 3D shape). This means the shape of the active site may be incorrect, so the enzyme doesn’t work.
mutations in body cells
If a mutation occurs in a body cell it may lead to cancer.
mutations in gametes
If a mutation occurs in the egg or sperm it can lead to the offspring having a genetic disorder
mutations (in introns - non-coding pieces of DNA)
mutations in introns could cause the gene to be switched on or off when it should not be. This would affect how genes are expressed.
E.g. we might get no protein synthesis when we need it, or we could get proteins made unnecessarily. Why may this be bad?
- We may lack a protein we need
- We may make a protein when we don’t need it. If this is an enzyme it
may catalyze a reaction we don’t want to happen, or simply waste energy making proteins we do not need.
introns
Introns are non-coding pieces of DNA – but they are used in gene expression. They ‘switch on’ genes, meaning they control whether their respective exon needs to undergo protein synthesis.
Cystic fibrosis - what is it?
Cystic fibrosis is an inherited disorder of the cell membranes. It results in the body producing a lot of thick sticky mucus in the air passages (which makes breathing difficult). It also results in movement of substances in and out of a cell becoming more difficult
Cystic fibrosis - type of allele
Its caused by a recessive allele.
Therefore, the presence of two alleles will be needed
to cause the characteristic to be seen in the
phenotype . because its recessive, people with only 1 copy of the allele won’t have the disorder - they’re known as carriers - they carry the faulty allele, but don’t have any symptoms.
Polydactyly - what is it?
Polydactyly is an inherited disorder where a baby’s born with extra fingers or toes. It doesn’t usually cause any other problems so isn’t life-threatening.
Polydactyly - type of allele
Its caused by a dominant allele and so can be inherited if just one parent carries the defective allele. The parent that has the defective allele will have the condition too since the allele is dominant.
Types of embryonic screening - Pre-implantation genetic diagnosis (PGD)
During IVF, it’s possible to remove a cell from each embryo and analyse its genes. Embryos with ‘healthy’ alleles would be implanted into the mother - the ones with ‘faulty’ alleles destroyed.
Types of embryonic screening - Chorionic villus sampling (CVS)
CVS involves taking a sample of cells from part of the placenta (where the baby’s umbillical cord joins with the mothers uterus) and analysing their genes. The part of the placenta that’s taken and the embryo develop from the same original cell - so they have the same genes. If the embryo is found to have an inherited disorder, the parents can decide whether or not to terminate (end) the pregnancy.
Embryonic screening
Embryonic screening is a way of detecting inherited disorders in embryos