meiosis Flashcards
why are gametes described as haploid
this is due to them only having one copy of the chromosomes
first stage of meiosis
the DNA unravels and replicates so there are two copies of each chromosome, called chromatids
second stage of meiosis
the DNA condenses to form double-armed chromosomes, each made from two sister chromatids. The sister chromatids are joined in the middle by a centromere
meiosis I
first division - the chromosomes arrange themselves into homologous pairs which are then separated, halving the chromosome number
meiosis II
the pairs of sister chromatids that make up each chromosome are separated (centromere is divided)
what is produced at the end of meiosis
four haploid cells that are genetically different from each other
chromatids crossing over
during meiosis I, homologous pairs of chromosomes come together and pair up
the chromatids twist around each other and bits of chromatids swap over
the chromatids still contain the same genes but now have a different combination of alleles
two ways during meiosis that lead to genetic variation
crossing over of chromatids
independent segregation of chromosomes
what occurs during the independent segregation of chromosomes
each homologous pair of chromosomes in your cells is made up of one chromosome from your mum and one chromosome from your dad
when the homologous pairs are separated in meiosis I, it’s completely random which chromosome from each pair ends up in which daughter cell
so the four daughter cells produces by meiosis have completely different combinations of those maternal and paternal chromosomes
outcomes of mitosis
produces cells with the same number of chromosomes as the parent cell
daughter cells are genetically identical to each other and to the parent cell
produces two daughter cells
outcomes of meiosis
produces cells with half the number of chromosomes as the parent cell
daughter cells are genetically different from one another and the parent cell
produces four daughter cells
what increases the rate of mutation
mutagenic agents such as ultraviolet radiant and ionising radiation
what is substitution
one or more bases are swapped for another
what is deletion
one or more bases are removed
what is addition
one or more bases are added
what is duplication
one or more bases are repeated
what is inversion
a sequence of bases is reversed
what is translocation
a sequence of bases is moved from one location in the genome to another
this could be movement within the same chromosome or movement to a different chromosome
what happens if the amino acid sequence changes
a polypeptide may change the tertiary structure of the protein, which could mean that it doesn’t work properly
what controls the transcription of genes
controlled by protein molecules called transcription
how do transcription factors work
they move from the cytoplasm to the nucleus where they bind to specific DNA sites near the start of their target genes
they control the expression of the gene by controlling the rate of transcription
activators - stimulate or increase the rate of transcription
repressors - inhibit or decrease the rate of transcription
how do activators work
example - they help RNA polymerase bind to the start of the target gene and activate transcription
how to repressors work
example - they bind to the start of the target gene, preventing RNA polymerase from binding, stopping transcription
how can oestrogen initiate transcription
it is a steroid hormone that can affect transcription by binding to a transcription factor called an oestrogen receptor, forming an oestrogen-oestrogen receptor complex
the complex moves from the cytoplasm into the nucleus where it binds to specific DNA sites near the start of the target gene
the complex can act as an activator of transcription, such as helping RNA polymerase bind to the start of the target gene
