Topic 3 Flashcards
Ways in which a prokaryotic cell is different from a eukaryotic cell
- Cytoplasm that lacks membrane bound organelles
- No nucleus. Instead has circular DNA and plasmids. The DNA is not associated with a histone protein
- 70s ribosomes rather than 80s ribosomes
- Cell wall containing the glycoprotein peptidoglycan
How is DNA organized within a bacterial cell?
- Large circular DNA
- Rings called plasmids
- Not within a nucleus, it floats freely around the cytoplasm
- The DNA is not wound around a histone protein
What are the additional prokaryotic cell structures?
Plasmids which are small loops of DNA
Capsule/slime capsule which protects bacteria from drying out and for protection against immune system cells
Flagellum which enables the cell to move
Pili which are thread like structures that help bacteria to attach to other surfaces
Mesosomes are infolded regions w many functions like dna replication, cell wall formatio and aerobic respiration
Describe how scientists can tell a cell is a eukaryotic organism and not prokaryotic
There are membrane bound organelles such as nucleus, golgi apparatus, rER, sER, vesicles
Eukaryotic cells have 80s ribosomes [size of ribosomes larger]
DNA is linear and its associated with a histone. The DNA is enclosed within a nucleus.
They do not contain plasmids
Describe what happens to the lysosomes once their contents have been digested
- It fuses with the cell surface membrane
- Then it releases the contents of the cell
Describe the structure and function of the lysosome
Round membrane-bound organelle with no clear internal structure
It contains digestive enzymes which digest invading cells or break down worn components of the cell
Describe the structure and function of the centrioles
Small hollow cylinders made of microtubules
Found in all animal cells but only some plant cells
Involved in spindle organization/separation of chromosomes during cell division
What are the functions of the golgi apparatus
Modify proteins
Formation of vesicles like lysosomes or secretory vesicles.
Concentrating the protein/glycoproteins made
Describe the structure of the golgi apparatus
Membrane bound flattened sacs
Formed by the fusion of vesicles from the ER.
describe the protein movement within the cell
- the nucleolus makes the ribosomes for protein synthesis in the rER
- the nucleolus makes the mRNA which is needed by ribosomes to make proteins. this is transcription of DNA to mRNA
- the mRNA leaves through the nuclear pore
- the ribosomes on thr rER make the proteins
- the rER processes the proteins. the protein moves through the rER assuming 3D shape en route
- vesicles are pinched off the rER and they contain the protein
- vesicles from rER fuse to form the flattened sacs of the golgi apparatus
- proteins are modified in the golgi apparatus, preparing the proteins for secretion
- the vesicles are pinched off the golgi apparatus and they contain the modified protein
- the vesicle fuses with the cell surface membrane, releasing protein, like extracellular enzymes.
structure and function of the rER
- its a system of interconnected membrane-bound flattened sacs
- formed of folds of membrane continuous with the nuclear envelope
- ribosomes are attached to the outer surface
- role of rER is to process proteins made on ribosomes.
- the proteins made on the ribosomes are transported through the ER to other parts of the cell
structure and function of ribosomes
- made of RNA and protein
- can be found free in the cytoplasm or attached to the rER
- they’re the site of the latter part of protein synthesis
- 70s ribosomes are found in prokaryotes, mitochondria and chloroplasts. but 80s ribosomes are found in eukaryotes
structure of nucleus
- relatively large
- enclosed by a nuclear envelope composed of two membranes perforated by nuclear pores
- contains chromosomes and a nucleolus
- the chromosomes are made of DNA and contain the genes that control the synthesis of protein
function of mesosomes in prokaryotic cells
- theyre infolded regions in the plasma membrane of some prokaryotic cells
- functions include aerobic respiration, cell wall formation and DNA replication
Describe the structure and function of the smooth endoplasmic reticulum
System of membrane bound flattened sacs with no ribosomes
Synthesizes and processes lipids and steroids (such as reproductive hormones)
what happens during interphase?
- new cell organelles are synthesised and DNA replication occurs.
- by the end of interphase, the cell contains enough cell contents to produce two new cells
- cell increases in mass and size
- it consists of three phases: G1, S, G2
- G1: some point in this phase, a signal is recieved to tell the cell to divide again. Gap 1 phase. its the gap between the previous cell division and S phase.
- S phase stands for synthesis of DNA where the chromatin is doubled and the DNA is replicated
- G2: where the cell continues to grow and the new DNA thats been synthesised is checked
what happens during prophase
- chromosomes condense, becoming shorter and thicker, with each chromosome visible as two strands called chromatids
- the two strands are identical copies of each other
- they’re effectively two chromosomes joined at one region called the centromere
- microtubules from the cytoplasm form 3D structure called the spindle
- the centrioles move around the nuclear envelope and position themselves at opposite sides of the cell, forming the two poles of the spindle.
- they organise the spindle fibres and the spindle fibre form between the poles.
- the nuclear envelope breaks down into small vesicles which signals the end of prophase and the start of the next stage.
what happens during metaphase
the chromosome centromeres attach to spindle fibres at the equator. when this has been completed, the cell has reached the end of metaphase.
what happens during anaphase
- the sister chromatids separate at the centromere.
- spindle fibres begin to shorten
- the chromosomes are pulled to opposite poles by the spindle fibres which shorten
- this phase ends when the separated chromatids reach the poles and the spindle breaks down.
what happens during telophase
- the chromosomes arrive at opposite poles and begin to unravel.
- the nuclear envelope reforms so the two sets of genetic information are enclosed in separate nuclei
- the spindle fibres break down
how is a sperm cell specialised for it’s function
- it is motile (can move)
- it has a flagellum, powered by energy released by the mitochondria, which allows it to swim towards the egg cell
- it has an acrosome in the head of the sperm. the acrosome contains digestive enzymes to break down the protective glycoprotein layer surrounding the egg cell so that the sperm cell can fertilise the egg.
- contains many mitochondria which provides energy from respiration so that the flagellum can move back and forth for locomotion
how is an egg cell adapted for it’s function
- cytoplasm containing a store of energy which provides the energy for the dividing zygote after fertilisation
- jelly like coating that changes after fertilisation which allows it to form an impenetrable barrier after fertilisation to prevent other sperm nuclei entering the egg cell
what is the acrosome reaction
- front of sperm (acrosome) touches the zona pellucida of the egg.
- acrosome swells and bursts, releasing digestive enzymes which break down zona pellucida of the ovum
what is the cortical reaction
- vesicles inside egg called cortical granules fuse with the cell membrane and releases their contents, causing change in surface layer of the egg, prevents other sperm from entering.
steps of fertilisation
Sperm reach ovum
Chemicals are released from cells surrounding ovum, and triggers acrosome reaction
Acrosome swells, fuses with sperm cell surface membrane
Digestive enzymes are released from acrosome
Enzymes digest through follicle cells and through zona pellucida surrounding ovum
Sperm fuses with ovum membrane
Sperm enters ovum
Enzymes released from lysosomes in ovum thicken jelly-like layer, preventing entry of other sperm
Nuclei of ovum and sperm fuse.
describe the steps of crossing over in meiosis
- homologous chromosomes pair up and are in very close proximity to each other, known as bivalents
- non-sister chromatids cross over, getting entangled and they join up at certain points, called chiasmata
- the entanglement places stress on the DNA molecules, resulting in a section of chromatid from one chromosome to break and rejoin with the chromatid from the other chromosome- known as recombination
what is independent assortment and how does it work?
the production of different combinations of alleles in daughter cells due to the random alignment of homologous pairs along the equator of the spindle during meiosis I
the homologous chromosomes pair up and are pulled towards the equator of the spindle
- when they pair up, they can be arranged completely randomly and the orientation of one homologous pair is unaffected by the orientation of any other pair
- the chromosomes are then separated and pulled apart to different poles.
- to work out the no of diff possible chromosome combinations, the formula 2 to the power n can be used, where n is the no of chromosomes in a haploid cell.
describe the steps of meiosis
- interphase: dna replication and growth of the cell
- meiosis I.
this is like mitosis, where it results in the production of two diploid daughter cells, they are the same as mitosis phases. in meiosis I, there is also crossing over that occurs. - meiosis II.
two diploid daughter cells divide in order to produce a total of four haploid daughter cells.
a) prophase II: nuclear envolope breaks down if it was remade after telophase I. nucleolus disintegrates, chromosomes condense and spindles are made
b) metaphase II: chromosomes line up in centre and bind to spindle fibres at centromeres. chromatids of each chromosome is independently assorted
c) anaphase II: spindle fibres pull chromatids to opposite ends of cell,, and centromere divides
d) telophase II: two nuclear envelopes develop to form two haploid nuclei. the two cells then divide by cytokinesis to produce 4 haploid daughter cells
Polygenic
controlled by a number of genes, at different loci (generally continuous)
Epigenetics
the study of changes in organisms caused by modification of gene expression
how does each gamete only recieve one allele of each gene
- the homologous chromosomes carrying alleles for each gene are separated
- sister chromatids containing the copies of the alleles are also separated
- spindle fibres pull chromosomes to opposite sides of the cell
egg cell - how its specialised
- Are much larger than sperm cells as most of their internal space contains food to nourish a growing embryo
- Have follicle cells that form a protective coating
- Have a jelly-like glycoprotein layer, known as the zona pellucida, that forms an impenetrable barrier after fertilisation by a sperm cell has occurred, to prevent other sperm nuclei from entering the egg
describe how cells become differentiated
- Under certain conditions, some genes in a stem cell are activated, whilst others are inactivated
- mRNA is transcribed from active genes only
- This mRNA is then translated to form proteins
- These proteins are responsible for modifying the cell (e.g. they help to determine the structure of the cell and the processes that occur within the cell)
- As these proteins continue to modify the cell, the cell becomes increasingly specialised
- The process of specialisation is irreversible (once differentiation has occurred, the cell remains in its specialised form)
what is a transcription factor
- protein that controls the transcription of genes by binding to a specific region of DNA
- ensure that genes are being expressed in the correct cells, at the correct time and to the right level
- Transcription factors that increase the rate of transcription are known as activators (help RNA polymerase to bind)
- Transcription factors that decrease the rate of transcription are known as repressors
explain role of centrioles in sperm cells following fertilisation (egg cells do not contain centrioles)
- is source of centrioles in zygote
- so spindle fibres can be synthesised
- so the fertilised egg/zygote can divide by mitosis
describe how process of fertilisation results in formation of zygote from gametes in humans
- contact between sperm and secondary oocyte results in acrosome reaction
- meiosis is completed
- cortical reaction takes place
- fusion of sperm’s nucleus with the nucleus of ovum
how does meiosis produce new combinations of alleles in gametes
- crossing over swaps genes between homologous chromosomes
- independent assortment, the random movement of homologous chromosomes to poles
