chapter 6 p2 Flashcards
Metaphase:
During metaphase the chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate, and then held in position.
Anaphase:
The centromeres holding together the pairs of chromatids in each chromosome divide during anaphase.
The chromatids are separated - pulled to opposite poles of the cell by the shortening spindle fibres.
The characteristic ‘V’ shape of the chromatids moving towards the poles is a result of them being dragged by their centromeres through the liquid cytosol
Telophase:
In telophase the chromatids have reached the poles and are now called chromosomes.
The two new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them.
The chromosomes start to uncoil and the nucleolus is formed.
Cell division - or cytokinesis, begins.
Cytokinesis:
Cytokinesis, the actual division of the cell into two separate cells, begins during telophase.
cytokinesis Animal cells:
In animal cells a cleavage furrow forms around the middle of the cell.
The cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells:
cytokinesis Plant cells
Plant cells have cell walls so it is not possible for a cleavage furrow to be formed.
Vesicles from the Golgi apparatus begin to assemble in the same place as where the metaphase plate was formed.
The vesicles fuse with each other and the cell surface membrane, dividing the cell into two (Figure 15).
New sections of cell wall then form along the new sections of membrane (if the dividing cell wall were formed before the daughter cells separated they would immediately undergo osmotic lysis from the surrounding water).
Mitosis and Genetic Replication in Normal Cells:
Normal cells have two chromosomes of each type (termed diploid) - one inherited from each parent.
During mitosis the nucleus divides once following DNA replication.
This results in two genetically identical diploid daughter cells.
What happens in sexual reproduction:
In sexual reproduction two sex cells (gametes), one from each parent, fuse to produce a fertilised egg.
The fertilised egg (zygote) is the origin of all the cells that the organism develops.
Gametes must therefore only contain half of the standard (diploid) number of chromosomes in a cell or the chromosome number of an organism would double with every round of reproduction.
What is meiosis:
Gametes are formed by another form of cell division known as meiosis.
Unlike in mitosis, the nucleus divides twice to produce four daughter cells - the gametes.
Each gamete contains half of the chromosome number of the parent cell - it is haploid.
Meiosis is therefore known as reduction division.
Homologous chromosomes:
each characteristic of an organism is coded for by two copies of each gene, one from each parent.
Each nucleus of the organism’s cells contains two full sets of genes, a pair of genes for each characteristic.
Therefore each nucleus contains matching sets of chromosomes, called homologous chromosomes, and is termed diploid.
Each chromosome in a homologous pair has the same genes at the same loci.
Alleles:
Genes for a particular characteristic may vary, leading to differences in the characteristic, for example blue eyes and brown eyes.
The genes are still the same type as they both code for eye colour but the colour is different, meaning they are different versions of the same gene.
Different versions of the same gene are called alleles (also known as gene variants).
The different alleles of a gene will all have the same locus (position on a particular chromosome).
As homologous chromosomes have the same genes in the same positions, they will be the same length and size when they are visible in prophase.
The centromeres will also be in the same positions.
The stages of meiosis:
meiosis involves two divisions:
Meiosis I
Meiosis II
Meiosis I
the first division is the reduction division when the pairs of homologous chromosomes are separated into two cells.
Each intermediate cell will only contain one full set of genes instead of two, so the cells are haploid.
Meiosis II
the second division is similar to mitosis, and the pairs of chromatids present in each daughter cell are separated, forming two more cells.
Four haploid daughter cells are produced in total.
Meiosis 1
Prophase 1
During prophase 1, chromosomes condense, the nuclear envelope disintegrates, the nucleolus disappears and spindle formation begins, as in prophase of mitosis.
The difference in prophase 1 is that the homologous chromosomes pair up, forming bivalents.
Chromosomes are large molecules of DNA and moving them through the liquid cytoplasm as they are brought together results in the chromatids entangling.
This is called crossing over (Figure 3).
Metaphase 1:
Metaphase 1 is the same as metaphase in mitosis except that the homologous pairs of chromosomes assemble along the metaphase plate instead of the individual chromosomes.
The orientation of each homologous pair on the metaphase plate is random and independent of any other homologous pair.
The maternal or paternal chromosomes can end up facing either pole.
This is called independent assortment, and can result in many different combinations of alleles facing the poles (Figure 5).
Independent assortment of chromosomes in metaphase 1 results in genetic variation.
Anaphase 1:
Anaphase 1 is different from anaphase of mitosis as the homologous chromosomes are pulled to the opposite poles and the chromatids stay joined to each other.
Sections of DNA on ‘sister’ chromatids, which became entangled during crossing over, now break off and rejoin - sometimes resulting in an exchange of DNA.
The points at which the chromatids break and rejoin are called chiasmata.
When exchange occurs this forms recombinant chromatids, with genes being exchanged between chromatids.
The genes being exchanged may be different alleles of the same gene, meaning the combination of alleles on the recombinant chromatids will be different from the allele combination on either the original chromatids (Figure 4).
Genetic variation arises from these new combinations of alleles - the sister chromatids are no longer identical.
Telophase 1:
Telophase 1 is essentially the same as telophase in mitosis.
The chromosomes assemble at each pole and the nuclear membrane reforms.
Chromosomes uncoil.
The cell undergoes cytokinesis and divides into two cells.
The reduction of chromosome number from diploid to haploid is complete.
Meiosis II
Prophase 2:
In prophase 2 the chromosomes, which still consist of two chromatids, condense and become visible again.
The nuclear envelope breaks down and spindle formation begins.
Metaphase 2
Metaphase 2 differs from metaphase 1, as the individual chromosomes assemble on the metaphase plate, as in metaphase in mitosis.
Due to crossing over, the chromatids are no longer identical so there is independent assortment again and more genetic variation produced in metaphase .
Anaphase 2:
Unlike anaphase 1, anaphase 2 results in the chromatids of the individual chromosomes being pulled to opposite poles after division of the centromeres - the same as in anaphase of mitosis.
Telophase 2:
The chromatids assemble at the poles at telophase 2 as in telophase of mitosis. The chromosomes uncoil and form chromatin again.
The nuclear envelope reforms and the nucleolus becomes visible.
Cytokinesis results in division of the cells forming four daughter cells in total.
The cells will be haploid due to the reduction division.
They will also be genetically different from each other, and from the parent cell, due to the processes of crossing over and independent assortment.
crossing over
independent assortment
Erythrocytes or red blood cells:
have a flattened biconcave shape, which increases their surface area to volume ratio.
This is essential to their role of transporting oxygen around the body.
In mammals these cells do not have nuclei or many other organelles, which increases the space available for haemoglobin, the molecule that carries oxygen.
They are also flexible so that they are able to squeeze through narrow capillaries.
Neutrophils:
a type of white blood cell
play an essential role in the immune system:
They have a characteristic multi-lobed nucleus, which makes it easier for them to squeeze through small gaps to get to the site of infections.
The granular cytoplasm contains many lysosomes that contain enzymes used to attack pathogens.
Sperm cells:
are male gametes
Their function is to deliver genetic information to the female gamete, the ovum (or egg).
Sperms have a tail or flagellum, so they are capable of movement and contain many mitochondria to supply the energy needed to swim.
The acrosome on the head of the sperm contains digestive enzymes, which are released to digest the protective layers around the ovum and allow the sperm to penetrate, leading to fertilisation.
Specialised plant cells:
Palisade cells:
present in the mesophyll, contain chloroplasts to absorb large amounts of light for photosynthesis.
The cells are rectangular box shapes, which can be closely packed to form a continuous layer.
They have thin cell walls, increasing the rate of diffusion of carbon dioxide.
They have a large vacuole to maintain turgor pressure.
Chloroplasts can move within the cytoplasm in order to absorb more light.
Root hair cells:
present at the surfaces of roots near the growing tips, have long extensions called root hairs, which increase the surface area of the cell.
This maximises the uptake of water and minerals from the soil.
guard cells:
Pairs of guard cells on the surfaces of leaves form small openings called stomata.
These are necessary for carbon dioxide to enter plants for photosynthesis.
When guard cells lose water and become less swollen as a result of osmotic forces, they change shape and the stoma closes to prevent further water loss from the plant.
The cell wall of a guard cell is thicker on one side so the cell does not change shape symmetrically as its volume changes.
