DNA, genes and chromosomes

Question 1

gene?

Answer 1

.short section of DNA coding for a polypeptide and functional RNA
- polypeptides make proteins so genes determine proteins of an organism
- location of a particular gene on a chromosome= locus

Question 2

allele?

Answer 2

diff version of same gene

Question 3

chromosome?

Answer 3

. how DNA is stored
- humans have 23 pairs
- pairs of matching chromosomes= homologous pairs- exactly same genes, but might have diff alleles

Question 4

how is DNA stored in eukaryotes?

Answer 4

. stored as chromosomes in nucleus
. linear shape
. DNA slightly wound around proteins called histones, to fit in nucleus as chromosomes = nucleosome

Question 5

how is DNA stored in prokaryotic cells?

Answer 5

. DNA in chromosomes
. shorter DNA molecules and circular
. not wound around histone- supercoils to fit in cell

Question 6

genetic code features?

Answer 6

. degenerate
. universal
. non overlapping

Question 7

start/stop codon?

Answer 7

. start= 3 bases at start of a gene on DNA so copied onto mRNA as well. initiates translation
. stop=final 3 bases on end of DNA so copied onto mRNA. doesn’t code for an amino acid so no comp anticodon w a particular amino acid so causes ribosomes to detach so translation stops

Question 8

Degenerate?

Answer 8

. 20 diff amino acids that genetic code is able to code for
. 64 combos is more than needed to code for 20 amino acids so each amino acid coded for by more than one triplet of bases so degenerate is a feature

Question 9

Universal?

Answer 9

. same triplet of bases code for same amino acid in all organisms so universal
. ADV as gene engineering is possible

Question 10

Non-overlapping?

Answer 10

. each base in a gene is only part of one triplet of bases that code for one amino acid so each codon read as discrete unit
. ADV as if theres a point mutation, only effects one codon so one amino acid

Question 11

what are introns and exons?

Answer 11

. intron= section of DNA that doesn’t code for amino acids therefore polypeptide chains. Only found in eukaryotic DNA. these get removed, spliced out of mRNA molecules
. exons= sections of DNA that code for amino acids

Question 12

genome and proteome?

Answer 12

. genome =organisms complete set of DNA in one cell
. proteome= full range of proteins in one cell
- genome should never change
- proteome constantly changing depending on which proteins needed
- genome of organisms varies a lot e.g bacteria have 600,000 DNA base pairs, humans have 3 billion

Question 13

RNA?

Answer 13

.Polymer of a nucleotide ( monomer) formed of a ribose, nitrogenous base and phosphate group.
.The nitrogen bases are adenine, guanine, cytosine and uracil. RNA polymer is a short polynucleotide and single stranded.
.The function of RNA is to copy and transfer genetic code from DNA in nucleus to ribosome. Some RNA also combine with proteins to create ribosomes

Question 14

3 types of RNA?

Answer 14

mRNA
tRNA
rRNA

Question 15

mRNA?

Answer 15

. Messenger RNA is a copy of a gene from DNA
. Creates in nucleus and leaves to carry copy of genetic code of one gene to a ribosome in cytoplasm
. Short lives as only needed temporarily to help create a protein
. It’s single stranded and every 3 bases in sequence code for specific amino acid = codons

Question 16

Why is mRNA needed?

Answer 16

. DNA too large to leave nucleus and could be damaged by enzymes so destroying genetic code permanently. mRNA is shorter as only length of one gene so can leave nucleus through pores.

Question 17

tRNA?

Answer 17

. Transfer RNA found only in cytoplasm
. Single stranded, but folds into cloverleaf shape, held by H bonds
. Function= attach to one of 20 amino acids and transfer amino acid to ribosome to make polypeptide chain. Specific amino acids attach to specific tRNA molecules and this is determined by 3 bases found on tRNA which are comp to 3 bases on mRNA= anticodon

Question 18

rRNA?

Answer 18

. Ribosomal DNA makes up bulk of ribosomes by combining with a protein

Question 19

Differences between DNA and RNA?

Answer 19

. DNA is larger, rna is only length of one gene
. DNA is double stranded, rna single stranded
. DNA has thymine rna has uracil
. Polymers of dna is deoxyribonucleic acid, polymer of rna is ribonucleic acid

Question 20

Proteinsynthesis?

Answer 20

. Proteins created on ribosomes in 2 stages:
- transcription= one gene on DNA copies into mRNA
- translation= mRNA joins with a ribosome and corresponding tRNA molecules brings specific amino acid the codon codes for

Question 21

Transcription?

Answer 21

. comp mRNA copy of one gene on DNA created in nucleus
. mRNA is shorter so can carry genetic code to ribosome in cytoplasm to enable protein to be made
Process:
- DNA helix unwinds to expose 2 strands
- one chain of DNA acts as a template to make mRNA
- to unwind double helix and break H bonds, enzyme DNA helicase used
-free mRNA nucleotides align opposite exposed comp DNA bases
-enzyme RNA polymerase joins RNA nucleotides to create new RNA polymer. One entire gene is copied
- mRNA modified and leaves nucleus through nuclear envelope pores

Question 22

Pre- mRNA + mRNA?

Answer 22

. Pre mRNA has to be modified to become mRNA that’s ready to leave nucleus and take part in translation
. Introns spliced out by a protein called splicesome. Exons left behind ( coding regions)

Question 23

Translation?

Answer 23

. Polypeptide chain created using both mRNA base sequence and RNA
Process:
- modified mRNA leaves nucleus and attaches to a ribosome in cytoplasm
- ribosome attaches at start codon
- tRNA molecule w comp anticodon to start codon aligns opposite mRNA, held in place by ribosome
- ribosome moves along mRNA molecule to enable another comp tRNA to attach next to codon on mRNA
- 2 amino acids that have been delivered by tRNA molecule joined by peptide bond. Catalysed by enzyme and requires ATP
- continues until ribosome reaches stop codon at end of mRNA molecule. Stop codon doesn’t code for an amino acid so ribosome detaches and translation ends
- polypeptide chain now creates and enters Golgi body for folding and modification

Question 24

meiosis

Answer 24

. type of cell division
. 4 genetically diff daughter cells produced
. involves 2 nuclear divisions and creates 4 haploid daughter cells from a single diploid parent cell

Question 25

how is the genetic variation introduced?

Answer 25

. 2 mechanisms:
- independent segregation of homologous chromosomes
- crossing over between homologous chromosomes

Question 26

independent segregation?

Answer 26

. in meiosis 1, homologous pair of chromosomes line up opposite each other at equator of cell
. its random which side of equator the paternal and maternal chromosomes from each homologous pair lie
. these pairs are separated, so one of each homologous pair ends up in daughter cell
. this creates a large number of possible combos of chromosomes in daughter cells produced
. calculated using 2n. n= number of homologous pairs

Question 27

crossing over?

Answer 27

. homologous pairs line up opposite each other at equator in meiosis 1, parts of chromatids can become twisted around each other
. this puts tension on chromatids so pairs of chromatid break so new combo of alleles in gametes

Question 28

meiosis vs mitosis?

Answer 28

. meiosis has 2 nuclear divisions
. haploid cell
. introduces genetic variation
. mitosis has one nuclear division
. diploid cells
. creates genetically identical cells

Question 29

what causes genetic variation? - summary

Answer 29

. random fusion of gametes at fertilisation
. independent segregation/ assortment in metaphase 1
. crossing over and recombination in prophase 1

Question 30

Full process of meiosis?

Answer 30

First division:
. Prophase- chromosomes become visible. Homologous chromosomes pair to form bivalents which eventually appear 4 stranded, each chromosome consisting of 2 chromatids. Nucleolus disappears. Spindle apparatus forms
. Metaphase- bivalents at equator of spindles. Centromeres attach to spindle fibres
. Anaphase- chromosomes move to opposite poles
. Telophase- spindle apparatus breaks down. Nuclear membrane may reform. Cytoplasmic division may occur or may be delayed until end of meiosis
Second division:
. Prophase- chromosomes reappear as 2 chromatids joined at centromere. Spindle apparatus forms
. Metaphase- chromosomes at equator of spindle. Centromeres attach to spindle fibres
. Anaphase- centromeres divide. Chromatids move to opposite poles
. Telophase- spindle apparatus breaks down. Nuclear membrane re forms around each nucleus. Cytoplasm divides.

Question 31

what is non disjunction?

Answer 31

. chromosomes/ chromatids don’t split equally during anaphase so gamete won’t have correct number of chromosomes

Question 32

two types of effects non disjunction can have?

Answer 32

. changes in whole sets of chromosomes so triploid gamete - polyploidy
. changes in number of individual chromosomes so one more/ less chromosome- aneuploidy

Question 33

what is polyploidy?/

Answer 33

. 3 sets of chromosomes rather than 2
. mainly occurs in plants
. fatal in humans- zygote won’t develop into foetus

Question 34

process of polyploidy?

Answer 34

. 23 homologous pairs
. each pair double due to DNA replication in interphase
. non disjunction in meiosis 1- all chromosomes fail to separate equally
. normal division in meiosis 2 - chromatids separate equally
. diploid gamete made
. fertilises with haploid gamete so changes in whole sets of chromosomes occur

Question 35

process of aneuploidy?

Answer 35

. non disjunction in meiosis 1 - one chromosome doesn’t separate equally
. meiosis 2 - chromatids separate equally so haploid cells made but missing chromosomes/ 1 extra