Cell Structure, Reproduction and Development flashcards

Topic 3: Cell Structure, Reproduction and Development

Pathway of a secreted protein in a eukaryotic cell

Transcription in the nucleus; mRNA leaves via a pore → translated on rER ribosomes; polypeptide folds in the lumen → transport vesicle to the Golgi → modified and packaged into secretory vesicles → exocytosis at the cell surface membrane.

rER vs sER

rER: flat sacs with ribosomes; folds and processes proteins. sER: tubes with no ribosomes; lipid synthesis and storage.

Functions of the prokaryotic capsule, plasmid, flagellum and pili

Capsule: protection, helps adhesion. Plasmid: small extra DNA loop, may carry resistance genes. Flagellum: movement by rotating. Pili: attachment; some transfer DNA.

Magnification and resolution

Magnification = size of image ÷ size of real object. Resolution = the smallest separation at which two points are seen as two. Electrons have a much shorter wavelength than light, so electron microscopes resolve much finer detail.

A cell image is 52 mm across and the real cell is 8 µm. What is the magnification?

52 mm = 52 000 µm. Magnification = 52 000 ÷ 8 = ×6500.

Linked genes and recombinant frequency

Linked genes have loci on the same chromosome and are inherited together unless crossing over separates them. Recombinant frequency = recombinant offspring ÷ total offspring; a smaller value means the loci are closer together.

How does meiosis produce genetic variation?

Crossing over between non-sister chromatids at chiasmata in prophase I; independent assortment of homologous pairs at metaphase I (2ⁿ combinations, n = haploid number); then random fusion of gametes at fertilisation.

Acrosome reaction vs cortical reaction

Acrosome reaction: enzymes released from the sperm acrosome digest the zona pellucida. Cortical reaction: the egg releases cortical granule contents, altering the zona so no more sperm can bind (the block to polyspermy).

Double fertilisation in flowering plants

The pollen tube grows down the style and enters the ovule through the micropyle; the generative nucleus has divided into two male nuclei. One fuses with the egg nucleus (diploid zygote); the other with the two polar nuclei (triploid endosperm nucleus).

Mitotic index and time in mitosis

Mitotic index = cells in mitosis ÷ total cells counted. Example: 36 of 240 = 0.15 (15 %). If the cycle lasts 20 hours, mitosis lasts about 0.15 × 20 = 3 hours.

Mitosis vs meiosis

Mitosis: one division; two diploid, genetically identical cells; for growth, repair and asexual reproduction. Meiosis: two divisions; four haploid, genetically different cells; homologues pair and cross over.

Totipotent vs pluripotent stem cells

Totipotent: can form every cell type plus extra-embryonic tissue (placenta), so a whole organism. Pluripotent: can form all body cell types but not extra-embryonic tissue. Cells of the blastocyst inner cell mass are pluripotent.

How does a cell become specialised (differential gene expression)?

All cells from a zygote have the same genes. A signal or transcription factor activates certain genes, which are transcribed and translated into proteins that determine the cell's structure and function. Other genes stay unexpressed.

Alternative splicing

Different exons are kept or removed from the pre-mRNA, giving different mRNAs and so different polypeptides from one gene.

DNA methylation vs histone acetylation

Methylation: methyl groups added to cytosine, often at promoters, reducing gene expression. Histone acetylation: loosens chromatin and raises expression; deacetylation lowers it. Neither changes the base sequence.

Multiple alleles vs polygenic inheritance

Multiple alleles: three or more alleles of one gene in the population, such as ABO (Iᴬ and Iᴮ codominant, Iᴼ recessive). Polygenic: several genes with additive effects, plus environment, giving continuous variation.